I have never heard of anything like that before. Do you mean that the horse has a bit/bridle issue with his head or that he runs in the pasture fine but when he is caught and tacked up, he suddenly turns up lame?
Well we were talking about my horse that appears to be lame in his shoulder, but the only sign is head bobbing, his stride is perfectly even and he tracks up really good.
And yes, he loves to burn around his paddock so I tend to think it can't be that bad if he's doing that. Meaning galloping around, bucking (four feet off the ground) and falling over like a complete idiot he's not the most coordinated of horses lol