what are your stft and ltft at idle/1500/2500/3500 rpms?if high at idle and get better at higher rpms than vacuum leak.if good at idle and get worse under higher rpms than possible dirty maf or low fuel pressure.maf should be able to put out over 4 volts under a snap throttle....but need a meter with fast enough capture capabilities min/max glitch capture or a labscope.have you looked at 02 readings at idle.....towards low end?have you checked fuel pressure at idle and also snap throttle?may be a weak pump.do you have a fuel pressure gauge?if you have a scan tool that can do a ford koer or key on engine running test and fails for 02 lean condition and suspect a vacuum leak.should have shown up in a smoke test.what size engine/model and year?any tsbs on this problem?what type of scan tool do you have?dirty injectors/fuel filter?