It got to about 98 degrees in town today and the a/c shut off. When I go home I left it running and hosed down the condenser with no joy. Then I opened the hood and sprayed water on the metal lines running to the compressor and it instantly came back to life.

My temp is running at about one and a half marks below 210 degrees since I replaced the thermostats. About 18 months ago the dealer put in a new compressor and accumulator.

Seems to me the over pressure/over temperature sensor tripped. :confuzeld Should I consider going to a better fan and clutch to move more air at a lower temperature?

I am no expert, but one of your AC pressure switch may be broken. Usually, the one attached to the accumulator.

At least that is better then what happened to me... My high pressure switch blew off causing compressor oil to go everywhere :(. Happened probably when i had my defroster on since I haven't used AC since early last year... I think my snapring rusted or something though :(

My A/c didnt work at idle. I did the harmonic balancer and front crank seal the other day and noticed a bunch of crap stuck in the rad. I pulled the rad, cleaned all the coolers and radiator and WOW!!!! what a difference! the A/C is freezing cold again, and the truck stays much cooler.

I had no idea so much crud could accumulate in there!

My A/c didnt work at idle. I did the harmonic balancer and front crank seal the other day and noticed a bunch of crap stuck in the rad. I pulled the rad, cleaned all the coolers and radiator and WOW!!!! what a difference! the A/C is freezing cold again, and the truck stays much cooler.

I had no idea so much crud could accumulate in there!

I need to do that too. I'm pretty sure thats why my AC is not working very well. Cant hardly run the AC and pull any trailer, even my 1500# utility trailer. Ac gets cold only after running down the highway for a few minutes. Spraying cold water from the garden hose in the grill brings down the temp of the coolant and the AC. AC pressures dropped 50% also. This tells me there is an airflow issue, either due to the clutch and fan or the condenser, radiator and oil coolers are plugged. I would say its the latter of the two. Still need a duramax fan and clutch probably and the 97+ water pump and maybe dual tstat crossover. I noticed also that 97+ 6.5 has a different clutch and fan than a pre 97 6.5 does. So thats another possible upgrade for us 92-96 6.5ers and we shouldnt have to worry about trimming the duramax fan to keep it from rubbing on the shroud or anything.

Not only does airflow effect engine temp but it effects the AC alot also. The heat from the cab has to go somewhere.

As for the OP, a dirty rad, condenser, plugged up radiator, faulty fan clutch or anything like that will cause a rise in AC pressures while the AC is on. If it got hot enough it could hit the high limit switch. As i said a few minutes ago, using one of those "rain wand" water hose attachments to spray water in the grille dropped my AC pressures from 55 low side to 35 and 250 high side to 125 high side. That's quite a drop, and was only on an 80 degree day. Think of it nearly 20 degrees hotter. I could see it tripping the high pressure switch.

Today it was 80 degrees and the a/c worked fine. When I sprayed water on the metal lines before it helped the compressor to kick in. Was it just a coincidence? Maybe I could wrap the cold line in insulation to keep it from absorbing heat?

When outside temps go up so does the pressure in your AC system. I charged mine back in the winter and now that summer has a arrived I had an overcharged system. I let a little pressure of and the system works great now. BTW when it's hot outside your low pressure port should read no more than 55 deg. F. Duh - 55psi

The high pressure switch will kick off the system if the refrigerant gets too hot. Sounds to me like there was inadequate cooling, ie. condensor plugged. Could also be moisture in the accumulator can that is freezing. that would require draining the system and replacing the dryer element. My suggestion would be to clean the condensor, radiator and the coolers that are in front of them really well. This took me a couple hours including drain and pulling the rad. My labour is also way cheaper than the A/C shop.
Even if that doesn't fix your problem, the coolers and rad will work more efficiently, and your truck will thank you for it.