I had a real similar problem, but I don't have a VCT Zetec. For me it turned out my MAF was dirty and needed cleaning. The "great" guys at the ford dealership did not latch my Air Cleaner back down and dirt got in and mucked up the MAF.
I used electrical contact cleaner to fix the problem.
http://www.contour.org/ubbthreads/showflat.php?Cat=&Board=trouble&Number=47402&Forum=trouble&Words=MAF&Match=Entire%20Phrase&Searchpage=2&Limit=25&Old=6months&Main=45997&Search=true#Post47402
MAF == Mass Air Flow sensor. It is right past the air box on the way to the throttle body (before the resonator.) It has the electrical connections on it.
I was also throwing a CEL. Are you getting a CEL?
Rick