Have you checked whether or not there is wear on the sprockets? You don't mention whether you are running a freewheel or cassette. Or if a cassette, whether it is UG or HG. This would also be helpful information.

I would remove the sprockets from the freewheel body or disassemble the sprockets on the cassette. Most times, excessive wear is seen first on the spoke side of the sprocket, and with the exception of the largest sprocket, can't be seen unless the sprockets are disassembled. When you do post pictures in order for us to assist in determining.