Toyota Repair: 96 avalon a/c, toyota avalon, compressor clutch

Question
I have a 1996 toyota avalon with 195k miles. Never had any problem with the a/c until recently. It will stop blowing cold as if you had turn the a/c off, but the compressor clutch is still engaged. It will work when you first start driving the car, but sometime in the day it may stop. It don't stop cooling everyday, just sometimes. When it does stop cooling I have to let the car sit overnight and the next day it will cool again. It doesn't happen every day.
I have checked the low side pressure. When the car is cooling it runs 33psi and when it's not cooling it looks as if it's in a vacuum and it reads below 0psi.
I was wandering if the expansion valve would have to do anything with this problem and where is it located?
If you need any more info let me know.
Thanks for your time.
Buzz

Answer
The low side pressure with the compressor on is normal, what is not normal is the reading going to zero when the compressor stops, the static pressure should actually go up to about 80-90 psi and then drop when the compressor engages,a compressor going into vauum or zero pressure normally indicates that the receiver drier may be clogged or it could also be an expansion valve problem,the pressure will be low on both sides,from the information I have it could be either one, I would start with the drier it's easy to replace and not very expensive, if you're not sure what it looks like,  get one from the parts department first, it should be mounted on a bracket in front of the condenser in the high side pressure line.