Air conditioner blows warm and cold
 

2007 Buick Lucerne w/3.8 V6, 96K miles
Very recently, the A/C started blowing warm and cold. I live in TX, so it can get a bit warm in the afternoon.
I had the freon evacuated and re-filled. It was a bit low. But it has always been able to cool the car during the afternoon drive home, even in 110 deg weather.
Just this last couple weeks, it has started blowing warm. It seems to do this on acceleration, possibly related to low vacuum. I had an older GMC van that had a vacuum leak that caused the same sort of thing to happen. But this doesn't seem to be as consistent. If I sit in traffic for a significant amount of time, it will start warming up. After a while in normal freeway speeds it will cool down significantly. When it blows cold, it is good and cold.
Could it be a vacuum leak? I thought these cars were all electric controlling the doors and such. I also disconnected the negative pole on the battery for about an hour to force a computer reset. That didn't seem to help.
Suggestions?

Thanks,
Bryan

If you have a leak, you will fin the compressor oil wherever it is leaking from if it's in a spot easily found. it could be the evaporator it's self if a rock hit it hard enough in the right spot. Just out of curiosity is the radiator fans on when you are sitting still in traffic?

Ha... Get this. The front motor mount on the v6 was shot - rubber was ripped out. The engine was rotating enough to pull on the wiring connected to the AC temperature sensor. Everytime the engine rocked, the AC would cycle. It never knew what the temp was...

They fixed the frayed wires and replaced the lower front motor mount. I replaced the upper torque mount (10 minutes.) Now the AC works fine.

Damndest thing I've ever seen, and I grew up on a farm!

Just to show everything in life has a purpose. At least if didn't involve refer recovery of the A/C system.