I just had the same issue, after replacing the belt (yes, from Beoparts) on the Beocord 5500 it every time stopped playing/ff/rew, after 4-5 seconds. when checking, I discovered that the belt was pulling itself up to the side of the pulley of the motor (almost rolling off). The pulley was to deep to the motor. after pulling it back up so that the metal center and the pulley were aligned (and in line with the capstan wheels), the belt kept on the correct place. This did the trick in my case.
Probably I pushed the pulley in while handling the tape mechanism.
