Toyota Repair: 89 Toyota Camry A/C, toyota camry, time span

Question
My 89 Camry a/c began to give intermittent trouble recently.  It would blow cold on start up -work for 10-20 minutes then blew hot. Sometimes I could turn on and off at switch and get system to blow cold again. Took to mechanic who said it had no pressure on the low side.  Took to another shop who argued with me that it was obviously "Broken" now whether it blows cold sometimes or not and said expansion valve was clogged and recommended changing that and the evaporator core and conversion to 134A, but upon installation of the valve and inspection of the core (I asked him not to replace if not warranted), he decided to not replace the core.  The expansion valve installation resulted in correction of pressure but after 20 minutes, the pressure drops on both sides and the a/c blows hot again.  Now he wants to replace the compressor.  I suspect that something else is wrong, since right now he is experiencing the same problem I reported when I brought it in.  Any other valves or sensors to check before I shell out another $400?  Your help is sincerely appreciated!  

Answer
It's obvious that the expansion valve was not the problem and I don't think that the compressor is the problem either, the pressures on both the high and low side of the system dropping in that time span indicates a large leak, if the compressor stopped working the line pressures would actually increase if there is no leak.
The best advice I can give at this time is to take the car somewhere else and get another(hopefully)honest opinion, try to have a toyota dealer check it if possible or a repair shop that specializes in air conditioning, I don't have much faith in whoever you are dealing with now, it sounds like he just wants to try replacing this and that at your expense, this is not the way to repair a problem whether it's an a/c issue or anything else.