94s10LS

I rust recently bought a 94 S-10. Great truck all around, the motor and tranny were rebuilt 30-40,000 miles ago. It runs, and rides great. The only rotting is one small hole on the bottom of the tailgate, no biggie though. Really the only problem is that the speedo sticks a bit, once you get over twenty or twenty five you have to tap on the dash glass to get the actual reading. Can anyone tell me what it is and how I can fix it? Thanks.