Try something like a KMC Z51 chain (renamed the Z8.1 GY/GY). That cleared up a problem on another bike with friction shifting and older Suntour freewheels.
I prefer the KMC Z72 (not sure what they call it now) for my index shifting setup with Suntour GPX and SunRace freewheel. But that chain clattered a bit more with Suntour Alpha freewheels in friction mode.
The main difference between the two chains appeared to be the rivets -- plain peened vs mushroom head. Not sure about the internals. I think both had identical 7.1mm pin length and shared the same Missing Link connectors.