I have an 85 911 cab that has been running hot recently. The temp needle has steadily increases during startup as normal. Once I reach normal running temp the needle starts bouncing up and down in about 1mm increments. This continues as I drive and at a stop light and continues to register hotter and hotter. I have not let the needle pass the second tick on the temp gauge for fear that I'm running too hot. I'm getting good oil pressure and when I check the oil level it's halfway between high and low marks on the dip stick. All this happened pretty quickly in 80F weather in about 15 mins of city driving. Could this be a bad temp sender unit or bad engine thermostat ?
Have a look at the temp sensor on the right side of the engine,