Air Conditioner Compressor

I was told this weekend I need a new compressor. I have a 10 year old Pontiac Bonneville whose a/c works intermittently. It generally works better when the weather is cooler than really hot (I live in Phoenix, so cooler is low 90’s, hotter is 110). It sometimes will start cooling at once, sometimes after running a half hour, sometimes not at all. Supposedly pressures are good, fuse and relay good, good voltage, ground circuits good.

Any opinions?

I am not ready to condemn the compressor yet. If the pressures are good, the compressor must be doing something. I think someone needs to check the pressures when the air conditioning is cooling and again when it isn’t.
When the air conditioning isn’t cooling, is the compressor clutch engaged? This is easy enough to check visually. If the clutch isn’t engaged, then check for power at the clutch. If there is power, then the clutch is suspect. This can be replaced without replacing the entire compressor.
I had a similar problem with the air conditioning on a 1990 Ford Aerostar. The problem turned out to be a bad fan clutch and not enough air was being pulled through the condenser. On your Bonneville, check to see that the radiator fans are working.