Change the chain to a IG. UG are usually found on the lower end components like 6 speed freewheels.
The profiles of the side plates of a UG, HG and IG chain are slightly different. I've never even used a UG chain for years, I've always gone for the HG.
That plus you say it's a IG cassette points to the chain being the incorrect type. There are IG specific chains you know?
Something like this should do the job:
http://www.chainreactioncycles.com/Mode ... ModelID=81
The profiles of the side plates of a UG, HG and IG chain are slightly different. I've never even used a UG chain for years, I've always gone for the HG.
That plus you say it's a IG cassette points to the chain being the incorrect type. There are IG specific chains you know?
Something like this should do the job:
http://www.chainreactioncycles.com/Mode ... ModelID=81