Hotpoint TDC60 Intermittent Drum Rotation?

Hi,
My Hotpoint TDC60 tumble dryer heats fine but rotates briefly before it makes a short crackling noise (like a poor electrical connection) and then stops rotating. It still seems to be heating as it starts to smell a bit and if I hit it it starts rotating again. Sometimes it keeps on doing this and I have to give up but sometimes it seems to work okay.

Does anyone have any ideas as to what might be wrong.

Thanks, James.