Hope he feels better soon with the ulcer meds. It’s amazing the different way ulcers can manifest in their behavior.
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Ulcers can definitely cause the issues you’re seeing… jingles for him that he is feeling better soon 
If ulcer treatment doesn’t help, the next steps I would take is a very good eye exam (you probably will have to go to a vet college for this).
In my jumper he has some issues with his corpora nigra. On bright sunny days he sometimes spooks at jumps. Luckily it’s pretty manageable for him.
If that found nothing, I’d do two weeks of equioxx and see what happens.
Hopefully he will feel better once the ulcers are addressed. Unfortunately they can sometimes be a side effect of other issues. If you suspect PSSM2 it would be worth doing the DNA test.
Considering all the tests and remedies tried thus far, I’d think about this too.
Have you tried changing up this guy’s training routine? You might try more hacking and less arena work, say, or introducing basic dressage exercises, just to see what happens. Even if this is a sporadic pain issue, it certainly couldn’t hurt to work on relaxation and versatility, since psychological tension and repetitive motion both contribute to pain.
You could also use training activities in a systematic, diagnostic way, making it a point to see if anything in particular exacerbates the problem. Hill work? Work at speed? Tough terrain?
There’s nothing more frustrating than mystery lameness. Good luck to you, OP!
