Probably a tight link or two. Check the chain and make sure each link pivots freely on each roller. If one doesn't, use a chaintool to wiggle the pin back and forth until it does.

Another possibility is a tight spot in your chain (caused by an out-of-round chainring/spider, not uncommon). Check the chain tension through the entire pedal stroke. You should be able to easily lift the chain 1/2" in either direction at all points.