[924CarreraGTP]

I bet it's a bad thermostat. Are you sure it's only when the A/C is on? Have you let it idle with A/C off? My '83 did this too for a while. I pulled the thermostat and it was sticking. I put in a used one that I got for free out of an '85 928. It tested good and it runs exactly where my '84 runs now. A little bit over the first mark.