If it's a slight deviation I'd expect it might be something near to it that it a source of magnetism that's interfering with the pitch control, if it's speeding up and slowing a lot though then I've no idea
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.
take out the plate and clean the dust on the edge of the plate or inside the rim...i do leave my covers off and it tends to happen ...hope it can help ...also the little pole in the middle may need some oil ..i put a little oil on the pole...lubricate carefully and clean excess oil ...good luck