I have a late 80s Bianchi Volpe which came with Sakae Ringyo SX cranks and bb, SunTour barcon shifters and derailers, and SunTour chain.
The freewheel was aftermarket 7 speed and never rolled well so I replaced it with a Shimano 7 speed available at my LBS.
Everything was working well and the drive train was low noise and decently shifting despite the frankenbike nature and problems others have had, specifically that SunTour shifter throw and cage travel is not SIS compatible.
Recently I replaced the chain with a SRAM 9 speed based on reports that the narrower chain is even better on noise and can confirm, drivetrain is much quieter.
But now the chain skips under load. I know cog wear can be the source of this skip, but neither the chainwheels nor freewheel appear to have that high wear, and the original chain was only a few hundred miles past the Park Tool's ".75" indicator mark.
Attached is a picture of the filthy cranks (Chicago winter salt and sand included).
If I do need to replace the two largest chain wheels, must they be SR? Other brands seem more widely available, assuming BCD matches.