I'm only judging from zetec experience here, but I think this could be caused by any of a variety of things that cause a mild or moderate lean condition -- the most likely candidates being, I would guess, either a vacuum leak or a bad or dirty MAF. If it gets worse maybe you'll see a CEL.

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