A/C problem in '00 Blazer

I have a 2000 Blazer that has just developed a problem in the A/C.
The cooling is intermittent. Sometimes the compressor will engage immediately when the truck is started and put out cold air (28 - 32
degree air). Sometimes the compressor will NOT engage for 10 - 20 - 30 minutes and I will get hot air. Then, all of a sudden, I'll get cold air. It might stay cold for 5 minutes or it might stay cold for 30 minutes - and then turn hot again. There seems to be no rhyme or reason - sometimes it works and sometimes it doesn't.
I have charged it with refrigerant (134a with oil) and when it cools, it is REALLY cold.
With the hood up, I can see that the compressor clutch is not engaged when it is blowing hot air and, as soon as the clutch engages, I get cold air.
I don't know much about A/C systems but I'd like to get an idea as to what might be the cause before I commit to spend major bucks on a 10 year old vehicle.
Any suggestions?
TIA
Chuck
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wrote:

Like following accepted diagnostic procedures? Like checking the charge pressure properly? Like checking the low refrigerant level switch? Like checking the ambient temp thermostat? Like checking...
Yea, there are lot's of suggestions, but start with the simple ones. When it fails, make sure the low refrigerant switch is still closed, the ambient temp thermostat is closed, and work from there. Check voltages.
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