Ideal (Henrad) SE40FF boiler fan locking?

I have a Ideal SE40FF boiler. When the boiler is cold, the fan spins and the boiler runs just fine. However, once the boiler reaches temperature and turns off, it will not then restart due to the fan jamming. I have turned the boiler off and opened it up and the fan motor can be spun by hand. If I spin the motor and quickly turn the boiler back on, the motor immediately locks solid. Sometime it will even spin back a quarter of a turn and then lock. Any ideas anyone?