Experiencing similar issues with my '04.
Cruising level, or light downhill grade AC blows full force out vents (and cold).
under subtle (but constant) acceleration or slight uphill grade air flow from main vents appears to clamp off, and instead I can feel cold air (though not as cold) blowing from the defrost vents at base of windshield and from the floor vent.
This behavior reminds of how the vacuum actuators of old might have behaved if there was a subtle loss of vacuum due to leak and under hard accel.
When the air flow from the main vents stops, simply lifting your foot from the accelerator pedal for 1-2 seconds is sufficient to have the air flow return full force from the main vents.
Since the aux/rear AC unit upper/lower and blend doors are electric (and I'll just say it, that whole rear door actuator debacle is Class Action worthy. Machining gears out of "Captain Crunch" cereal would have been a more robust choice... but I digress).
Therefore I just assumed the front had a similar setup.
Edit (update)...
AC pressures all within normal ranges. Low pressure cut-off is not the cause of my problem. According to the Workshop manual most of the Control and Blend doors on my '04 are vacuum control motors and not electronic control. This, leads me back to my original gut feeling... vacuum leak. I will investigate the component mentioned (below) by Expedfan04, and also check a few other problematic vacuum lines to see if I can locate this leak. I also have to drop the headliner and reattach a disconnected driver-side front drain line from the sunroof (disconnected drain lines are real fun when it rains).
I just finished replacing a water pump on my wife's '99 Mercury Mountaineer 5.0l V8 (AWD), new hoses, pump, idler pulleys, serpentine belt, oil change, and general maint this last weekend so will give my knuckles a rest and try and tackle my ride's Gremlins this weekend.
