I'm surprised it works now with the Shimano cassette. Suntour used slightly different spacing in it's freewheels.
Sheldon Brown explained this
here:
Surprisingly, though, if you have Sun Tour 6-speed indexed shifting, you can convert to 7-speed using your present shifters! All that is needed is to replace your old rear derailer with any Shimano unit. Shimano vs Sun Tour indexing is different, but the mismatch in brands turns out to cancel out the mismatch 6 vs 7, so this actually works pretty well! The low gear position won't have a "click" in the shifter, but the derailer's low-gear limit stop will provide the "indexing" function for the low gear.