Sporadic Problem with Chevy Prizm Air Conditioner

I just had the A/C condenser replaced on my 1999 Chevy Prizm. Now it has an intermittent problem. Sometimes, when I start the car and turn on the A/C, the A/C will make rumbling / grinding / vibration sound. (I can even feel the vibration in the gas pedal.) The a/c will “click” on and off (you can hear the clicking from under the hood). And the RPM will vacilate up and down (while sitting at idle). If I try to drive while it’s happening, it will affect acceleration (causes the car to lose power and “lurch”).



In general, it happens when the car’s been sitting outside in the heat (above 85 or so), but hasn’t been driven in at least an hour.



When it happens, I turn the a/c off for a few minutes, and turn it back on, it’s usually fine. If not, I turn it off and back on again.



It seems to take longer to cool down than before the new compresser, but it still gets quite cold eventually.



Any clues? I took it back to the repair shop, but they couldn’t recreate it. (Sometimes, the car goes for days without the problem. Other times, it happens every time I get in the car.)