Ok, I have an '84 Schwinn Traveler 12 speed with Suntour ARX friction shifters. As far as I know the components are original. It was a gift so I can't be sure. My chain only slips on the 3rd largest sized cog/gear in the back. Here are some pics:

https://s755.photobucket.com/albums/x...t=PIC-0052.jpg
https://s755.photobucket.com/albums/x...t=PIC-0054.jpg

When I am pedaling it's like the chain wants to jump to a smaller sized cog/gear but never actually makes it and I can never trim up my gears on this cog but I can on all the others. This happens any time I am in this gear and never in any of the other gears. I thought that maybe I had a stiff link but when I checked there was no stiff link. I also checked to see if my chain has stretched and just needed to be replaced but it is also fine. I also checked to see if my front cogs were worn and they are not.

I just had the crank adjusted. It was making a tick tick clunk sound but now it is fine. It just needed some tightening. I don't know if this has anything to do with the chain/gear problem but thought it might be some helpful info.

Any info or advice would be greatly appreciated. I tried to be as detailed as possible. If more info is needed let me know. Thanks so much.

Either the gear is worn or the chain is stretched? How did you measure the chain?

Some people ride in one gear and one gear only, causing just one freewheel or cassette cog to wear out. You should see their face when you tell them they have to repalce a whole freewheel over one cog being worn, which is more cost effective then replacing just the one cog if a shop can find one freewheel cog then have the tools to change that one cog.

There's a chance that the previous owner used that cog much more than the others and therefore wore it down much more than the others. Take a look at the teeth on that cog and see if they are differently shaped or worn compared to the others.

The fix for this is to get a new freewheel and chain (switch one and not the other, the problem will come back). Will be about $30 from some place like Niagara.

I measured the chain by following this tutorial:

https://bicycletutor.com/chain-wear/

I used a tape measure and counted 12 complete links (one inner to one outer). The 12th link lined up within the acceptable perimeters.

Is it possible to have one gear that needs replacing and none of the others? So if the gear is worn I will have to replace the freewheel/cassette ( I don't know which I have)?

99% chance you have a freewheel. Swapping this out is a 10 minute job at the LBS. You can do it yourself if you buy the correct freewheel tool for your particular brand of freewheel. You can change just the one worn cog, but it's rarely worth it to take a freewheel apart to get at the one cog.

Wow, thanks everyone. Disappointing but I guess it could be worse.