I had this same skipping chain problem before and it was caused by severely worn cogs. The gear teeth looked more like hooks when viewed from the side. It was a very old Bridgestone with 7 speed Shimano LX. The cassette was replaced and the LBS recommend that I replaced the chain as wear which showed signs of wear (stretched and deflected). However the problem persist and a week later, I had the LBS inspect the bike again. Seems like a 10-speed chain was used, and being narrower, it didn't "sit" properly on the cogs. Chain was replaced again (at no charge) and the skipping never occured again.