Toyota Repair: air conditioner, metal particles, compressor clutch

Question
A 1994 Toyota 4 Runner with 107,000 miles, currently in hot and humid Houston, TX.  The A/C works in the mornings and in cooler weather pretty consistently.  Afternoons in the summer, after sitting in the parking lot, it will run for about 5-10 minutes on the freeway and then the air temp warms up to a point where its not cooling at all (3- minutes).  When it works, it will freeze you out.  The Toyota dealer A/C Tech says sometimes the compressor is running when the cooling fails and sometimes it is not.  Also, when they start the system and monitor its performance, it seems to show a steady build up of pressure that they say is way to high.  In addition to apotential problem with the compressor, they think there might be some material plugging up a key orfice and\or influencing the function of the expansion valves.  The mechanics are not quite sure about a diagnosis that they like and don't want to start fixing or replacing items just yet. I has done this for about 6 months.
Any ideas?   
thanks--don

Answer
Ok, I'll try but if they are not sure about their diagnosis with all the equipment available in the shop I don't know if I can be of much help without actually seeing the pressure readings which are very important in diagnosing the system, you say the pressure is too high, I need to know which side is too high, the low side or the high side, and I need exact figures. There are just too many possibilities that could cause your problem.
There are several basic things that could be the problem:
The compressor is failing and has distributed metal particles into the system, causing the expansion valve and the receiver drier to clog up, this will cause high pressures.
The expansion valve failed on it's own and the compressor is OK.
The compressor clutch is failing.
Here is what I suggest you do:
Tell the guys at the dealer to remove all the freon and remove the compressor and check to see if there is a problem with it, with the lines disconnected and the plate removed, the compressor can be inspected to see if it is seizing or if there is any metal inside, if the compressor is good replace the expansion valve and the receiver drier, the compressor is the most important part of the a/c system so before doing anything at all to repair the problem they must be absolutely sure the compressor is good.