Strange re-occuring lameness!!!

My horse ends up lame about once a week only in the AM. His hind left pastern is usually nice and swollen and he tries to use only his other 3 legs. Definitely doesn't want anyone touching it. It ususally goes away within 6 hours (while I'm in school). Once it goes away he's perfectly fine until it happens again. He's in extremely light work, nothing stressful. And he's only 6, so I don't think age is a factor. Any ideas?

Is he stalled?

He's in a stall for about about 6 hours a day.