It seems as if your sprocket's and possibly the chain needs some maintenance.This can be done properly with the purchase of a chain cleaning box,in the box you put chain cleaning liquid in the top chamber secure the box to the chain turn the peddles and the liquid will start to go black,when this operation is done,take the box off and clean it out.Now add new oil into the box secure it to the chain and your chain will be coated as you turn the peddles.Follow the box instructions,buy the proper oil and you can do this yourself