'X' reg 1.8 petrol ford focus cuts out at low revs?

I have a 1.8 zetec ford focus, 2001 'X' reg.

It has an intermittent problem in that when I try and start the car it won't start unless I have the accelarator pressed. As soon as I release the accelarator pedal the car cuts-out.

It has been on a diagnostic machine but doesn't return any fault codes, a breather pipe from the rear of the engine block was replaced and the car was ok for 2 weeks but then the same fault has returned.

Anybody have any ideas please ?