Originally Posted by Arrowana

See those tiny little grooves on the top of the cog teeth? The chain is getting stuck on those. Many French cogs have this design, and they are all at risk of this happening. Throwing a modern 8-speed chain makes the problem even worse. Finding a chain that has the same width as the original will make it less likely to happen, but will not eliminate the problem. Your best course of action here is to replace the rear wheel with something that uses a standard freewheel or cassette.