Few quick notes.
Keeping your pulley, belt, and inner platter surface clean is very important on Thorens decks. To clean the pulley and the surface of the inner platter where the belt runs, take some rubbing (isopropyl) alcohol, higher % the better, and a que tip and scrub the pulley belt surface and the inner platter belt surface. Especially on the pulley, you will see black grime come off. For cleaning your belt, take a paper towel and put some warm water on it and with the paper towel between your index finger and your thumb, LIGHTLY pull the belt through the paper towel.
A good rule of thumb with these decks is that if you don't know if you have a genuine Thorens belt, just get one. The belt can make or break these tables. You can get them from Q-Audio and Vinyl Nirvana, both have websites.
It is possible also that you need to oil/lubricate your platter axel bearing and bearing well. A lot of people use different things, some use gun oil, some sewing machine oil, etc.
We would be able to give you some better advice if you post them pictures. But something you should especially take note of is if your belt is rubbing against the metal speed changing arm (that bastard metal arm next to your motor pulley). I've had this problem before and it is fixed with a combination of proper suspension setting and proper motor angle.