Hind-end flexing problems

[PTancredi@aol.com]

I have a 13 yr. old gelding that had about 1500 miles of endurance. He was 
born with long slopping pasturns in the hind end, but with riding and 
conditioning they have pick up rather well. Lately though when the shoer 
comes and holds his hind feet up to work on them, the horse has a hard time 
putting his feet back down and putting weight back on them. Actually it is 
more his left hind. When you ride him there is not really anything to notice 
but occasionally he will take a hip- hop step in the hind end. Is this a hock 
problem or a suspensory problem or does anybody have another idea.
Thanks, Pat