Sounds like the A/C system is loosing vacuum, either due to a bad check valve or a vacuum reservoir being to small.
When the engine is near full throttle there is no intake manifold vacuum to replenish the system, and most times with no vacuum the dash controls will default to defrost; then return to their settings when vacuum is restored.
Not sure where the check valve and reservoir are on your Ford, but the valve will be between the intake manifold and the reservoir in the vacuum line of course and it closes the line so vacuum wont bleed back to the motor at full throttle. The reservoir may be too small ( black plastic about the size of a soft ball ) I added another on my last tow vehicle for this reason. Cruise control on my P/U was using all the vacuum and starving the A/C.
Just re-read your post , I was assuming gas engine , the same problem is possible with a diesel , except there is a 12volt vacuum pump instead of engine vacuum , erratic operation of the 12v pump could cause these problems too.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.