1988 240 DL - Long story. Daughter called and said her car was overheating. Low on coolant. Water pump leaked so replaced water pump and threw in a new thermostat (195F). Car still overheats, she said that when she turned the heater on the gage went down. It only overheats sometimes. In this adventure replaced the thermostat again, radiator, fan clutch and a used gage and attentuation board. Had a shop test for a blown head gasket. Gage goes up to about 3/4 scale, sometimes for only a minute or two, sometimes it stays up a while. Sometimes the heater on affects it, sometimes not. Sometimes when going down the freeway sometimes when you slow down. Never consistent. I'm out of guesses. Thanks.