I have a 2004 Ford Excursion with the 6.0 powerstroke. Most the time, the AC works fine, but after a long drive in hot weather, the front AC barely blows out of the vents. The rear works fine, in fact, my wife and I try to point those towards us, but that is underwhelming.
I have already replaced the blower, which made no difference. In fact, I can hear the blower motor ramping when we crank up the fan. It seems as if the air is being caught within the dash. It doesn't make a difference whether we have it blowing at our feet, face, or defrosters. I don't even know if I could really take it in into a shop, since it works initially. We don't typically take it on long drives, unless we are camping, so troubleshooting is pretty difficult. One time, when it happened, we had to sit in a line waiting our turn for the RV dump, and it eventually came back.
None of my factory gauges indicate overheating.