Is this happening when you touch the pitch control? If so your slider/resistor is screwed. Its a reasonably simple job to do but does require soldering, parts should cost a few pounds.
You can get away with it for a bit maybe by putting some WD40 oil on it and working it in by moving the slider, much like you would with a buggered fader or x fader on a mixer. Dont put too much WD40 just a little.