I need answers

I know I probably sound completely, diagnosabl-y, bipolar when it comes to Finn. Last week after the horrible day I had with him I had a heart to heart with my trainer who basically told me that if I felt I was done, then be done. It's not worth the time and aggravation I am putting myself through if I wasn't having fun anymore. And that's what this is supposed to be, fun! But then she also said that she really does like Finn, thinks he's a good horse, thinks he really hates the winter, and that he is just going through a phase. She said that he's a smart ass and knows I'm afraid of him. But she also said that she can help me.

She rode him most of last week and last Friday we split my lesson up half and half with her riding the first half. All the horses were being brought in for dinner, it was about 15 degrees out, and there was just a lot of commotion. He spooked a few times, and I said to my trainer that Finn was all hers. She said put your helmet on, and GET ON!

Got on, and at first he had his head in the air, looking all around and I felt like he was a second away from either rearing or bucking. She told me to loosen my contact, talk to him, and scratch his neck. As soon as I started having a conversation with him, and laughing, his head dropped, he started chomping lightly on the bit, and sighed. We worked for about 40 minutes, mostly at the trot, changing directions often, and collecting and extending his stride, letting him stretch out his back. He felt phenomenal.

And for the past week, he's been pretty good. My trainer has been riding him quite a bit. This afternoon I had an early day at work so I headed up to the barn. The cold has broken a bit, and it was a sunny 30 degrees (feels like summer compared to 0 degrees). Brought Finn in, let him chill in his stall for a while in case he had to pee (I didn't want a repeat of my ride from two weeks ago). Put him on the cross ties and he was calm and perfect. Brought my saddle out of the tack room and he immediately started prancing around and pinning his ears. I could barely tighten my girth as he was so upset. I didn't even bother with a bridle and stuck him on the lunge with just his saddle. He was nervous and prancing on the walk to the indoor (a horse that was FINE 10 minutes before), and as soon as I let out a little bit of line he TOOK OFF.  A totally accurate description of how he acted was that he looked just like a bronco- rear, buck, rear, buck, over and over. He was not listening, he had this crazed look in his eyes, and he was moving so quickly that I was getting dizzy. He was making this horrible noise that sort of sounded like a squeal/groan, and from the center of his panic around me it sort of looked like his outside hind leg (right) was coming way far out from his body, when he would settle and canter it looked like his hind legs were cantering together, then he would kick one hind leg out and start cross-cantering again or racing. It was crazy.

My trainer says, always says, that he is testing me. This is not testing me. This has to be pain. Or am I crazy?!!!
1. This is a horse that is on ulcer maintenance/treatment. I also give tums pre-ride/lunge.
2. His grain was cut in half two weeks ago.
3. He is turned out 8 hours a day in a giant paddock with buddies.
4. He gets good quality forage all day long.
5. Coat and body condition look great. He has even muscle tone throughout his body.
6. He is on SmartCalm Ultra and has been for three months now.
7. He is in slow, top line-building work six days a week. He is asked to work long and low at least 30 minutes a day, with tiny bits of collection thrown in, and a small amount of cantering 20m circles both directions.
8. I had my saddle reflocked and refit two months ago, and get it done every three months.
9. He just had his teeth done two months ago.
10. He was just tested for lyme.
11. He had a massage last month (done monthly), and chiro 3 weeks ago.

I would think that being in this much work, in a frikken program, on little grain, and on SmartCalm Ultra he wouldn't be completely mental unless he was in some sort of pain, right?

Once I got him back in the aisle, on the cross ties, and saddle off, he was still tense but not as bad. Brought him back outside, and called the vet while sitting on one of the rocks in the paddock. Once I let Finn go in the paddock he never, ever comes over to me. This time he walked away and then decided to come back to me and hang out with his head draped over my shoulder. He is not a bad, mean, or angry horse. He is a sweet boy and I honestly think there is something wrong with him. I talked to his vet for about half an hour about what to do next. She will be up next week for back x-rays. Possible things that I've mentioned to her are:
1. Kissing Spine-- he seems to have all the symptoms. Unpredictable, explosive, difficult to girth, unwilling to work.
2. Cervical arthritis- neck arthritis? This is not something I know much about or have researched a lot, but some of his strange symptoms also seem to line up with this.
3. Hock or stifle issue-- with how his leg has been swinging out he may have something going on with his stifle, although it never looks like his stifle is actually locking. Hocks- last time he flexed 1 out of 5 and vet said eventually he will need them done. I really don't think it's his hocks, seems higher up.
4. Hoof angles- Finn's always had great hind feet (yay for something!), and I don't think his angles are off, or it is a hoof problem, as it does seem to originate higher up.
5. SI's-- maybe he needs his SI's injected again? Symptoms also line up with this. This time around though I will get his SI's injected with ultrasound assistance. Last time we did not inject using the ultrasound.
6. Ulcers- I have never gotten Finn scoped for ulcers. I have him on SmartGut Ultra, and have treated with 30 days of ulcergard. Could his ulcers be so bad that he needs more than what I've done? His symptoms seem more extreme than ulcers though.
7. EPSM/RER- Also something I don't know a lot about. I know that it causes extremely tight muscles (which Finn has throughout his entire body), but he's never experienced, to my knowledge, an episode of tying up. I also don't think this would cause his unpredictability/explosiveness.

Another strange thing that both me and my trainer have noticed is that he doesn't sweat. This entire winter, even after tearing it up on the lunge for 30 minutes, he won't even be warm. After a ride, his chest won't be warm or sweaty. It has been very cold, and he doesn't have a long winter coat this year, but he has always sweat before.

 Hopefully next week we actually get some answers. I know I posted this a few weeks ago....Finn on the lunge with my trainer before a ride. Notice the strange way he bucks/squeals. And, this was a good day. This is how he "warms up" every day. I don't mind a horse that bucks, acts a little naughty on the lunge when the weather is cold, when they've been stalled for a while, to get the sillies out. What Finn does though I feel is on a whole other level. He will buck/rear for 20 minutes while I say over and over whoaaaaaaaa. He is just SO nervous and SO on edge.