A/C Quiting

My bossman has a beautiful 01 Lariat CC/SB that he has a 4" lift and 35s on and when towing a load (doesnt have to be big).. Anytime above 70 mph, the A/C will quit blowing cold.. The truck has right at 100k miles.. It doesnt do it empty.. Blows cold air all day long, but with a load blows cold air all the way up to 69-70 mph.. 1 mph over its hot air?

I'm not big on guessing..
Any way you could have it scanned, and look at the throttle % data when it's acting up?

But if I were to guess, I'd tell you to have it charged, pull a shim out of the compressor hub, and replace the fan clutch.

My 02 quit blowing cold under a load about five years ago, pulled the shim and problem fixed. Turn the a/c on and key to run position to engage the clutch to make braking the bolt loose easier. Turn key off to separate clutch and remove shim and put clutch back together. Turn key on and tighten bolt.

Here I just posted the link on another thread here, here is a write up on checking to see about the gap that hawk eyes is talking about.....

http://springerpop.net/F350/air_gap.html

Definitely sounds like the compressor shim deal. Drove me freaking NUTS when mine did it.