Originally Posted by Venetian Fred

Have you checked the rubber pipe behind the inlet manifold.The one that goes to the pcv valve?
these sometimes split or even worse get blocked with gunk.
If theres no splits,whilst your engines running check to make sure the rubber pipes not constrained.
I had one in on a 02 plate not so long ago for a service that was running like a dog.On closer inspection the pipe was actually going flat when the engine was running.
you may as-well change the pcv valve whilst your at it.I think ford say 90k+ or something but it wont hurt it.
anyway i hope this helps.