If the problem is anything like the one I had on a Soundlab DL-P1R, it's a glitch with the electronic board that controls the pitch adjuster. Somewhere on the turntable there should be two small holes through which you can insert a jeweller's screwdriver and adjust the turntable's speed via a little potentiometer. The 45rpm one is probably glitching, though sometimes twisting the speed up then back down can fix it.
Jennyemily got a reaction from ZoeKirkRobinson in Speed Issues on SL 20