Sometimes the amount of collective chain/cog wear/miss meshing is smaller then you'll see. back in the early 1980s I worked at a shop that served many of the local firemen. There was one really big one who brought his bike in for service with a specific to a skipping problem. We did all the usual and installed a new chain and freewheel. We test rode it with good function. he picked it up and came back still with skipping. We went through at least 3 different freewheels and IIRC 4 chains, all new, before we came on the combo which he couldn't get to skip. Now to place a bit of perspective on this I'm only 66" tall and back then weighed maybe 125lbs. But my boss was the NYS road racing champ from a few years prior, stood 74+" tall and was still to be reckoned with in the local racing community. After the first come back he test rode every combo and each time we had to have the customer test ride it to see. After the second come back we did the rest of the combos in one day while the customer hug out, test rode and hung out while we changed something for the next test.

First make sure the other reasons for skipping are not present. Tight or twisted chain links. Broken or bent teeth. Proper high gear limit screw adjustment.

It is hard without a extremely worn cog to see wear enough to cause skipping. But the test to see is straight forward. Try a fresh chain. Then try a fresh freewheel. Then report back. Andy.