Truck Overheating

[Cheryl Newbanks <horsetrails@inficad.com>]

Ok this isn't exactly endurance related so answer in private please.  But
in a way it is because if I can't haul my trailer then I can't go to rides.
 My 1993 F350 1 ton dually Ford overheats when I am pulling a trailer, or
hauling hay, IF I have the ac on MAX, I can keep it from overheating for a
while with AC on norm if I slow down around 50 and or turn the ac off.
I've taken it to 2 ford dealers, both times drove in overheating and they
tested the clutch fan, ok, thermostat, ok, pressure is great!  Then they
tell me I'll just have to wait till it breaks to fix it!  WRONG: I don't
want to be stuck on the side of the freeway in AZ in the summer in 110
degrees with horses in the trailer.  
Has anyone else had this trouble and if so what was it.  It is now starting
to overheat without hauling anything if the AC is on.  I do notice some
belts squeeking like they are slipping when I start it.  Could it be that I
need new belts and that the belts slipping with the ac on puts too much
load on the engine?  All ideas are appreciated.
   
                                  Cheryl Newbanks 
~~~           ~~ ^ ^        SW Region
~~~\      _ ~~/ /\ /        Buckeye, AZ
       (   ) __    ) ' '        horsetrails@inficad.com
       //          \\ 
      //            \\
    **             **