Try a working hunter class with good solid rustics. Bet he'd go well if you attacked it like hunting. Also are you showjumping indoors? Try a big outdoor arena and have a strong canter round outside first.
Agree with lots of the hints - take a good size clip board to lean on. Plus can be useful to have something box shaped under that to raise the clip board off your lap. Don't get carried away watching!!
Not sure about KS. May be some muscle or ligament damage. Try a McTimoney practitioner. Sounds a bit like stringhalt. Interesting as I read yours and has made me post my issue - see intermittent near hind lameness post on 19th. Also, look on You Tube for vids of anything similar. Can be helpful.
Very occasional lameness in near hind - always on left rein in circle - looks as if the stifle locks for a few strides in transitions but then carries on. Also will sometimes just stop and put in massive buck as though trying to release something and then carry on - this is usually when on the...