I am getting my recently acqured Gitane Tour de France back on the road. I have purchased a Simplex Criterium to replace the incorrect Prestige that came on the bike. I am using a set of clincher wheels rather than the original tubular wheelset, at least for now.

Here is the problem! I temporarily set up a shift cable and went for a short ride. It shifts beautifully, much better than expected and I think I will really enjoy the bike. The only issue is that when in fourth gear the chain slips. First, second, third and fifth are just fine. I changed the freewheel and it still slips. The chain is brand new and not binding. It does not slip when on the large chainwheel, only when the front is on the smaller one. I am frustrated as I can see no visible wear on either of the freewheels, the chain seems tight when on that cog.

Am I missing something obvious? I would be grateful for any advice you can offer.

Thanks!