Oh, so it's not a post kegging thing. That's good. If it's an ale, it seems doubtful that it'd be a yeast issue. Not many ales produce diacetyl.
I had a friend who bottled a beer, it turned butterscotchy, then went away after a while in bottles. I think it was because the temps changed during fermentation a lot. I guess that's one benefit to bottling conditioning versus kegging...