Frank might be right, but at 15 years old, it may be time to rebuild the shifters. Branford Bikes used to have instructions online and parts, I don't know if they still do (Campagnolo stopped making spare parts for those old shifters a year or two back). It might be best to have someone rebuild the shifter, replacing all the worn parts; the labor cost would be cheaper than the third or fourth minimum S&H fee.