Update.....

This evening when the temperature got a bit more tolerable, I went out and switched wheels with another bike. No skipping of the chain in 4th. I put the Gitane wheel on the other bike-no skipping of the chain. I then switched freewheels between the two wheels, ending up with the freewheel from the non-Gitane bike on the Gitane. It skipped once with an extreme effort on the pedals, much improved from before.

My theory is that the new chain and the old freewheel would have worked just fine with a Campy or other derailleur, but with the Simplex they are a bit touchier. There is not as much chain wrap with the Simplex. I am going to try an older chain on the Gitane and I am hoping that will eliminate it completely.

The Gitane is not going to be a real high mileage bike, as my time is limited and divided among 4 other bikes. Hopefully this will eliminate the problem!