I know this thread is old but I was wondering if you ever found what the cause was. I have a 2008 V6 3.5L Chevy Impala and it does the exact same thing. Once I rev the engine above 4k rpm the AC starts cooling and works fine afterward even if you shut off the car and start it again. But if it sits for a while, then you have to rev it up to 4k again to get the AC working. I don’t have the tools to measure the pressures on the AC lines but it seems like it is the exact same problem you were having.