If it stumbles at the exact same throttle position every time no matter what - it's the throttle position sensor.
Otherwise there can be many possible causes, most of which relate to running lean. This is often caused by vacuum or intake duct leaks.
Tecsup95 :
In your case, since you seem to have a 99 Zetec, you may be having a VCT issue. If VCT is getting sticky it could cause this. Many 98's have had VCT problems, so it's about time for the 99's to start acting up.
