Technics SL-B3 turntable unstable playing speed

Helli, i own a Technics SL-B3 turntable  (very similar to SL-B2) and when i play a record the speed is unstable. Suddenly running fast during 1 second then  slow , and fast again and so on. I cannot have a stable speed. Have you an idea of how to fix this problem. Many thanks in advance for your help. Best regards