gonna go out on a limb here, and say that the factory fuel mapping may be what your seeing.
at idle (unless cold), there is VERY little demand for fuel, therefore the EEC will not give it much.
of course, as the rpm (and throttle position) increase, the fuel need increases, so more fuel, and more power avail.
since these things are supposed to pass emissions tests, and MOST driving is done at lower rpm, running with a leaner mix may be the way Ford designed it.
As a side note, if a chip is gonna be emissions legal (they gotta tell ya when its sold), its pretty limited as to what they can do with the fuel mapping, and remain legal ( assuming NO other parts are changed in the vehicle, such as different cats, ect).
you say the MAF is as clean as its gonna get.. what did you clean it with (some chemicals are bad for it). Also, it may just be going bad too!
The mixture guage you have.. how is it connected?? does it use its own O2 sensor, or piggyback onto the engines sensor, or some other way? Also, does it just say 'lean' and 'rich', or does it give an actual mixture measurement (numerical, as in 14-1 air/fuel)? If its just 'lean and rich', just what do them 'values' mean?? lean could be 15-1, and rich could be 13 to one.. both perfectly acceptable for most daily driver uses.. you actually have no way of know what the actual ratio is on them type guages.
Remember, 14.7 to 1 is what the makers usually strive for as ideal, but thats NOT for power reasons, but for emissions reasons.