A/C running hot until I 'rev' engine.

I've got a '95 and just had the compressor replaced at the end of summer '03.. and it ran fine for the 2 weeks left of that summer.. and the entire next summer('04). Now this summer it is blowing hot air when I start the car until I get off the 25 mph streets onto a street where my RPM's get above about 2000.

Is this because I"m low on refrigerant?
I bought a interdynamics 'charge & measure' set to check the pressure.
When I start the car and the A/C is blowing hot... the pressure is into the 'red' bar (about 70 - 80 PSI) on the gauge!!.. however if I then rev the engine... the A/C air starts to get cool.. and when I recheck the pressure.. it is in the middle of the blue zone (which states there is 'enough' refrigerant).
(Yes Max A/C and coldest setting.. and high fan)
Once the A/C starts blowing cold air.. it will do it for quite a while.. however.. sometimes like 15-20 minutes later it will start warming up again and I have to rev the engine for a second or two to get it to start blowing cold air.

so.. is that typical symptoms of low refrigerant?

thanks,
Tim

check your pressure switch.

Sounds like it isn't turning on the compressor properly.

also how hot is it outside. if its over 90 then that could be why. it needs to build greater pressure and remember that the engine cooling fan cools off the coil. if it can't pull the heat from the coil then it can't pull it from the cab.