Well I will wade in. LOL I think your are having an air flow issue. The evaporator is icing up. The reason it will cycle/run longer on the higher fan speed is the additional flow of warm air keeps it form freezing for a longer period. I would say that if you have had it to two dealerships they more than likely checked the Freon pressures and they should be correct.
So that would leave two common things to look for.
1) Just because your duct and evaporator are closed does not mean they can not be plugs/restricted by some thing. Real common one is for the evaporator drain to quit draining correctly and dust blending with water and filling the evaporator with mud/dust.
2) The evaporator drain not work even with clean ducts will back up on the evaporator and then it will freeze into ice and block the air flow.
I would make sure none of these two things are happening before I would look at a high pressure switch not working.
As for replacing the high pressure switch. You usually need to evacuate the system to do that on most systems.
There is a dead simple way to check for this. unplug the AC compressor. Makes yourself a pig tail with a switch in it. This needs to be long enough to reach the cab. You want the factory harness to feed your short harness and then your harness hooked to the compressor. This way you can turn off the power to the compressor. Run the tractor. At the first sign of you losing cold air turn the compressor off. If it the air gets colder for a while and then warms up then the compressor back on. If the cycle repeats then your system is not shutting the compressor off when it should. THIS IS ONLY true if the ducts/evaporator are clean AND the system is filled to the proper level of Freon.
P.S. The one dealership had a good idea and just did not tell you why. The only true way to 100% know if the system has the correct Freon level is to pump it all out and put the exact amount of Freon back in. Some systems are real sensitive to this. A few ounces low or high can cause the type of trouble your having.
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