Has it got a cone filter on it?

Had exactly this problem on a friends' N-reg RS2k and as soon as the original airbox went back on, the problem went away, even after doing a KAM reset, etc...

As above though, I would check the ISCV as well out of principle to make sure it is cleaned out as these are a known problem on ALL Ford models.

BTW: correct oil for these engines in 16V form is 5W-30, 10W-40 if it was an 8V as found in the Sierra and Granada models.