In my (zetec) experience, the surging idle seems to be something that can be caused by anything that throws the air/fuel ratio off. Some things to check are:
- clean the MAF element (and IAT while you're at it)
- look for vacuum leaks
- check air filter
- check fuel filter
- try injector cleaner

96 GL Zetec ATX, white with pinstripe, nickname " Sam Smooth "
mods so far: CTA intake with homemade heat shield, KVR drilled front rotors & carbon pads w/ 500° fluid
planned mods: exhaust (want to keep it quiet), e-ram (awaiting installation), diablo chip
involuntary mods: compression increased after head gasket failure