Funky AC Operation

The AC doesn't seem to operate well at idle. I've checked the low pressure side and it runs high, around 55-60 psi, but then very briefly and intermittently will pump down to normal range (35 psi.) For this test I used a refrigerant refill line w/gauge and put the AC on high fan, lowest temp setting and I can see the passenger side radiator/condenser cooling fan running even when the engine is stopped (and so I'm assuming this is the condenser fan.) The AC seems to work well while the car is traveling but only the initial period while at rest it doesn't seem to work. This is my brother in law's car and I'm looking it over for him but he says this is not normal for the car and that it would cool at idle. I have no idea where to go from here thinking it might be the AC ECU or a sensor feeding it. If anyone has any suggestions or insights where I should go from here I'd appreciate a shout.

Thanks,

Jon