Hey John: I am far from being an expert on this subject but if I am understanding you correctly you are saying that the chain does not actually "slide" over the cogs rather than engaging but instaed you are getting "skipping" where the chain does engage eventually but decides to give you a slight bit of jerking discomfort pedalling with a heavier amount of torque ie pedalling uphill (much like out old 45's use to do ont the turntable before we threw a couple of more pennies on top of the stylus) then I would have to agree with Earl. I know that on the lightweight alloy chain rings of my mountain bikes is that I have to watch chain stretch religiuosly so that I do not cause damage to my expensive (overly expensive) rear cassette. I am not sure if you tried this or not but here is what I would check first before I purchased a new freewheel. I would remove the chain and stretch it out flat on the bench. Take any ruler and line up any inch mark to the center of any one link making sure that the chain is perfectly straight. Then go down the ruler until you read 12" more. If the center of the link nearest the 12" mark is off that mark by as little as a 1/16" of an inch (some of us get carried away when it is a 1/32") then your chain is stretched and should be discarded immediately. The more you run a stretched chain over cogs the more damage to the entire drive train including the crankset rings. Like I said, this happens more often with the lighweight alloys (I think Earl mentioned that as well) then they do with the vintage freewheels and crank rings. When you want to save weight you are going to loose durability, has been my experience.
If after you find that your chain is indeed stretched and you replace it and it still skips then you have a freewheel that is shot. The teeth of the cogs would show some discoloration near the "ramps" and the dimensions of the teeth may be distorted not too different than what you may see at the dentist :) and a bad visit there.
I have also had personal experience with the chain just "running" or "sliding" over the drive train and that is a whole new type of game and I am sure that you have run across that yourself at one time or another. I am not sure how to correct that problem. I did manage to make it go away with a slight adjustment on the friction shift lever and an "in the saddle" chain alignment. I sure hope that helps. If you found out that your freewheel is history, I was just going to list a couple Shimano 3.3.3. that seem to be in pretty good shape and especially one that is 15/17/19/21/24 and weighs 455 grams (French threads). If you are interested let me know and I will get it to you as a late Xmas present. Certainly no where near the quality of a campy but might at least get you on the road again. Seeya
Jim Abt
Wausau, Wi. USA