wots wrong with my dryer?

ive got a hotpoint aquaruis tdl 31 tumble dryer, when i turn it on it heats up but doesnt spin, unless u spin it by hand then shut the door quick then it works ok, untill it spins the other way and stops so again spin it by hand and it spins on its own again? the belts fine it doesnt slip but i was wondering wot part i needed to fix this is it a new motor or sumthing simplier? any help would be brilliant thanks