[gearmesh, inc.]

When you are losing power, take a look at your fuel trim parameters to see if fuel is being added. You mentioned fuel pressure stays at a constant 60 psi like it is supposed to, so that should rule out the pump.

Fuel trim values typically run +/- 10% when things are running right. Since you have enough fuel pressure, a abnormally low reading MAF can under-report how much air is actually coming into the engine. As a result, fuel delivery won't be enough to mix with the actual air mass going in the engine. This causes the HO2s to read lean, so the PCM will add more fuel in order to get the HO2s to switch rich for a moment. The more fuel the PCM adds, the greater the positive fuel trim correction percentage. MAF sensors don't always set a code unless they crap out all the way.

Another way to check to see if a MAF is reading what it should is to watch how many grams/sec of air are reported from the MAF sensor when you run WOT to the top of the tach. The max number of grams/sec should be approximately 80% of the horsepower the engine is normally capable of. For example, if the engine is advertised at 300hp, you should see around 240 grams/sec of air coming into the engine at WOT redline.