Suntour shifters will react strangely with shimaNo der's. The mix of the two does not allow for exact indexing. However if you are going to replace your chan you may need to replace your cassette or freewheel as well, as the chain may skip around on gears as well. New chains on old cogs will do this sometimes.
Also there are still resouces for old school Suntour componets like
http://recycledcycles.com