For kicks try cleaning the motor pulley with isopropyl alcohol.
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Do not level the player by adjusting the towers (this will set the player in a different plan than the motor), instead make sure the platform is in level. Make sure the platter is in level with the subchassis (watch the three black inserts in each tower, they should all have the same hight). Is the platter riding to high? You can adjust the hight of the motor as well. Is the motor pulley at the right hight, this can be adjusted (can you somehow have bent this one)? Make sure the belt is not twisted. Sometimes I have had to adjust the springs to get the belt running nicely, not going up and down, but I have never had a problem with it slipping off.
Good luck, and sorry about the language.