Maybe a failing o2?
Before changing the primary o2 sensor (for the second time), my car would run fine for a bit, and then randomly run like garbage. It too was better with some throttle applied than with it being at idle or even just above.
Should throw a CEL, but mine didn't always...
Zetec....is that like Vtec or something???
Zetec Crew #103/ATX Faction #27 (retired)