Flymo PC330 compact ?

the belt comes off after several starts, the motor initially almost stops then the belt comes off the drum. I've lightly roughed the drum with emery paper installed a new belt obviously tightened belt as much as possible works ok for a while then belt flies off again,the machine has been thoroughly cleaned. A lawnmower centre has looked at it. I suspect the brake mechanism for the blade, 2nd year with this fault.