I agree with jim - fuel delivery is probably what's up. But I have also seem a maf become biased and read properly at lower air flows and wrong at higher. There is a PID called BARO that reads a frequency. Depending on the altitude at your area, it should be 150 to 160 Hz. BARO is read at 2/3 throttle or more and is based on the volumetric efficiency of the engine. The computer knows how much air should be pulled in at high throttle settings, and uses the maf to measure it. A comparison between actual flow and expected flow yields the barometric pressure. Then baro is used as a compensator in the fuel trim calculation. baro is a good indicator of a maf sensor's health. The MAF pid should also track with the tp voltage - within about 1/2 volt or so, depending on how fast you're moving the throttle.
Jim is also right about vacuum leaks not causing problems at higher speeds. Once you get above idle, and certainly by the time you're holding highway speed, the percentage of unmetered air becomes small enough that the normal long and short term fuel trim compensators will hide it. At idle, so little metered air is there that the unmetered can be up to about half the air entering the engine.... Anyway, more than you wanted to know, huh?
