500sl overheat issue

[lucky4teen]

I have a 91 500 sl that overheats only when the a/c is on. It runs roughly at 85 degrees celcius with no a/c then jumps to over 100 degrees with the a/c on but it never goes into the red area on the temp guage. I would say it gets to about 110 degrees max while driving the car.(at this point the coolant starts to boil out of the overflow container on the passenger side of the car) If I leave the car sit and let it run with the a/c on it never goes above 90 degrees. any ideas on what the problem could be? cooling fans are working and coolant levels are ok...

also if i pressure test the system I get air coming out of my overflow tank-looks like its between the plastic housing and the actual bottle itself, is this normal?