I had that with my 300tdi - it was the spade connector on the back of the alternater - kept falling off - I pinched it up with pliers to keep it in place. Just look at the alternater, at the back and low down - you'll spot the connector easily.

I've just solved this problem with my newly acquired TD5 ES - I removed the plastic end plate and lubed the square peg as already outlined by others, then got a pair of mole grips on the thing and turned it backwards and forwards until it was working perfectly but...I found the back wouldn't...