Hi, my chain skips, but only in the highest gear, and only under somewhat heavy load. It had this problem when I got it, but the 7th gear at the time spun free on the freewheel, so I assumed it needed to be replaced. I Put a different on on, as well as a chain that had worn with it, but still having problems. Any ideas?

You mean you put on a different cassette? Not sure what you are saying here.

I'm thinking your cassette has worn out. I had the exact problem and if everything else is in good shape, I would look into that. Be patient others will give their opinion too.

Yes, sorry. On the freewheel I took off, all the pawls on the 7th cog had been broken.

I recently replaced a freewheel, and chain on a different bike, so I put the two that had worn together on this one, to see if that helped. It didn't.

it wore down first.
freewheels are not rebuildable now, buy a new one and a new chain.