Had a similar problem with a 2.0 zetec, turned out to be a breather vacuum hose at the back of the engine ( under the throttle body) only available though dealers. Found a new one on good old ebay from a company called National Clearance Center, part of the Moray group. loads of parts available there. You can ring them too!!!
Just to add to my post above, the pipe had gone soft and flat at the right angle bend just at the back under the throttle body. I wrapped the replacement pipe with heat resistant tape hopefully to prolong the life.
Nope, problem is back so not the pipe! Might just try some injector cleaner and a new fuel filter before a diagnostics check.
Maybe the MAF after that.