I am not a mechanic... but I would shoot for the cat converter and maybe some vaccum leaks...

Do you have the Ford CD Manual ??? They have some flowcharts and they do have this problem and a flowchart for it..

** CT - USA **

* 96 Contour Gl 2.0
5 Speed Manual / Clarion CD Change
8" Subwoofer & Sony Amp (LOUD !!)
The Cleanest !!!

* 92 Mazda Protege
5 Speed Manual / 1.8

* 94 Yamaha Virago 535
(better than Viagra !!)