Some of these intermittent speed problems on turntables are due to the oxidation of the potentiometers that control the speed of the rotation. So if your turntable has a dial to control the rotation speed at 33 and one for 45 then try getting to the pot' under the turntable plinth and try squirting some contact cleaner in there (or better something like DeOxit which will clean and grease). This has worked for me quite a few times on a variety of turntable with erratic speed problems.
It's somethng fairly easy to try before you invest in a new motor (which still wouldn't fix the problem if it was due to oxidized pots).