Ok, I'll commiserate with you. I have an '81 240d that I bought last year that ran cold. I replaced the thermostat, and it reached 80C, then shortly started running cold again. I replaced the thermostat again, thinking it might have been defective, but that didn't fix it. So either I have happened onto a rash of bum thermostats (one OEM, one AutoZone), or something else is going on. How often do sending units go bad? Can they lose their calibration and just read low?