Your chain might have streched, but I doubt it. Your cogs might be too thin for the fat chain, but then again, I doubt it. If it skips on a regular basis (as in once every five seconds) then look out for stiff links. Maybe your cogs have worn a lot in the last 2 - 4 months and the chain hasn't "bedded in" yet. I think I had exactly the same thing happen to me though, It turned out to be caused by the chain being a bit slack, so take a look at that first , and if it's none of the above then I have no clue .