2005 SXT surging on highway with AC on
 

Car has 151,000 miles on it and runs fine without the AC turned on. When driving on the highway with the AC on the car seems to surge about every 20 seconds and it seems like the transmission slips at the same time. The RPM's go up about 300 during this time.
Did a tune up Sunday and checked AC low side pressure was at 40 @ idile. It only runs this way with the AC on. Please help.
All fluids are good and no engine codes

my 2000 feels like it surges with the a/c running at speed but its just normal I think being a 4 banger automatic 3 speed.

does it slip or feel like it slips with out the a/c on, going up a hill from stand still?

It happens on flat land and only with the AC running. It actualy feels like the transmission is slipping....it feels like the AC compressor is cycling on and off about every 30 seconds. Running at 70 mph the RPM's are at 2700 and when it revs up it jumps to 3000 for about 5 seconds and then goes back down.
I put the guages on it Monday and the low side is reading 40 psi at idle. The AC is cold but seems like it should be cooler. I live in Houston and it is around 100 degrees these days. It is strange that it only does it with the AC running.

I'm in Dallas, ya its hot alright. Least its not hitting 115+

since your trans isnt slipping with the a/c off then you dont have a trans issue but I would double check the fluid level once hot.

Is there any trouble codes?

To me it sounds normal, mine does that at highway speeds 60-75

Anyone else wanna chime in?