Only happens under saddle, and only at the walk. Horse will go very nicely most of the time, then randomly RF will feel like he stepped on a rock or something. Couple times owner felt/saw him thrust the foot out to the side when it happened. At first thought he was interfering, he’s a narrow guy, but she can’t really tell as it hasn’t been caught by someone on the ground yet. Working on correcting 0° angles on the fronts, no heat or swelling, negative to hoof testers, lameness only lasts for 1 step at a time, maybe like 3 times a ride? ( but not every ride). This is a young tb with no injury history and sound on flexions. Any ideas? I suggested she just have the vet ultrasound the whole leg 
Could possibly be cervical damage/arthritis! I have a retiree who has cervical arthritis and the best way I can describe it is occasionally his right front momentarily leaves the rest of his body and he will trip over it.
Equioxx, massage, and injections have helped if that’s what it is!
