I'm kind of going with DQ2B sounds kind of like a soft tissue thing. My mare had lameness issues off and on after comming back from Florida last year. vet thought it was her knee, injected it..no better, she'd seem totally sound, then do something silly in the field and then be almost 3 legged lame, then fine, then off. I didnt ride her durring this time, finally put her on total stall rest with short slow lungeing sessions each day (to keep her from tearing the stall
down. It has been over a year now but I started riding her again about 2 months ago and she has been perfectly sound (knock on wood).
Soft tissue damage requires lots of rest/time off.
there are all sorts of issues that can make a horse seem fine one day and lame the next. I'd keep looking into it.
As far as the boots, I dont think there is really a boot out there that is supportive enough to cure a lameness. But what I have found is that it can make you feel a whole lot better in a situation where you feel helpless. At least you feel like you are doing SOMETHING for poor horsie.