The problem is most likely a combination of wear on the cog and chain ring, this is most often caused by a streched chain. My KMC chain was streched a little over a 1/4th of an inch before I replaced it. <-- bad idea. 12 full links of chain should be exactly 12 inches.If there is over 1/16th of an inch of stretch, you should consider getting a new chain, 1/8 of an inch of strech, it should be replace ASAP. the cog/chainring should be replaced as well