Pitch problem. Help

Hi folks,

I've had my Goldring GR1.2 for a few months now. Wonderful entry level table. Sometimes it sounds so good it's downright spooky. Quiet, accurate, and lush.

Other times, I can hear fairly serious pitch inaccuracies - especially on guitar or piano, I can literally here the pitch wobbling up and down.

This seems to change day-to-day. Last night, I listened to my new Johnny Cash album and was stunned. This afternoon, the pitch wobble has reared its ugly head, and the magic is gone.

What's happening here? Why would this happen some times and not others, on the same record? And what can I do?

Many thanks for your help.