Zanussi TC470 tumble dryer:How can I stop the fan belt squealing? ?

About 3 months ago the fan belt on our Zanussi TC470 condenser tumble dryer began squealing and 'slapping'. This got worse as the machine got hotter. I replaced the fan belt with the specified size, and in the correct figure 8 format, but after about a week the problem started again. On a couple of occasions the fan belt has actually come off causing the heater to trip due to overheating. Fortunately I have been able to reset the heater cut-out so the dryer still works but the belt noise is a real pain. Any ideas on a solution please?