Temperature Guage

I have a 1998 M3 and recently I began to realize that it's taking a long time for the temparature guage to reach the optimal temperature. Winter just began, so I added anti-freeze a couple weeks ago and everything looked fine. Until a few days ago it started to happen again, The car runs fine, but the needle on the temp guage would fluctuate between the half-way mark (optimal) and the blue cold marker. Sometimes it would approach the half-way mark and then go back down to the tip of the blue mark. And this is after running the engine for nearly one hour at highway speeds. My heater was blowing kind of cool as well, although I had the temp at 78 degrees. Anyone ever came across this problem?

I'm guessing that the thermostat is not opening all the way or not staying open. When the engine starts to get hot the valve is supposed to open to circulate the antifreeze through the block, but when the cold fluid hits the valve it closes, not giving you nominal heat for the heater or to read on the guage. So I would replace the thermostat and start there.

That's exactly what happened to me. It was an open thermostat.