What is the shifter doing? One problem those shifters had was the lube getting gummed up, particularly if they sat for a while, and having difficulty going from a larger cog to a smaller one. My front shifter had this problem -- if you can blow it out with an air compressor, or can of compressed air, you might find it shifting better. The other potential solution is to hit it with WD40. I know, I know, but 2 years after thinking I needed to replace my front shifter on my beater bike, it is still working.

The tell-tale of that problem is that you can shift to a larger cog just fine, but it won't go to a smaller cog.

This may not resemble your problem -- if not feel free to ignore.