2003 3500 AC quits being cold after 2 hours driving

[agfckrowe]

Help! I drove our 2003 manual transmission 3500 for 3 hours at about 65-70 mph, everything's fine. Then after parking it for an hour, and driving for 2 hours, the AC quits putting out cold air and it seems like the fan is not pushing out the normal volume of air, even on high (its sounds like the fan motor is running on high, just not normal output both in terms of volume and temperature). This happened again one month later, with the same amount of time driving, same highway speeds, both times with outside air temps in the high 80s. When you living in the humid south like I do, I just can't live without AC. The only other post I could find (with a slow dial up PC) referred to the AC not working at an idle, not driving at 70 mph for 3 hours. Before I take it to the dealer, I'd like to have an idea what I'm looking for since our closest dealer is 1 1/2 hours away.

Thanks!

KR