Sounds like you may have a skid patch problem. What size chainring and cog are you running? If you're running something like 45x15, then you are skidding on the same part of the tire everytime. It might be something to look into.

Some guy had a decent site about skid patches and how to calculate them but now its 404'd. send me a personal message if you want to know more, or maybe you already do know, in which case, I hope I'm not being patronizing.