white knight wk427 drum only turns with a push?

my tumble dryers drum has stopped turning. if i take the lid off and turn it by hand then it goes and works fine. heats up and dries ok, but as soon as i stop it i have to manually get the drum turning again.. i thought it might have been worn brushes on the motor but there dosnt seem to be any. could it be the capacitor? any one in the know? thanks