Identify the trimmer potentiometers and spray thoroughly with contact cleaner. Use an appropriate screwdriver to turn the the potentiometer position back and fourth vigorously for a bit. This helps remove any dirt or oxidization.

Spray any push or slide switches with contact cleaner and operate them vigorously.

Let the board sit for a while, then reassemble the turntable and check for steady speed. If the speed is till unstable, or if you are not able to get a correct speed for 33 and 45rpm modes by adjusting the trimmer potentiometers, proceed further with this guide.