My Vehicle: '86 240DL. After an approximate 7-minute drive in 52-degree weather, I noticed the dashboard temp gauge was just entering the red. I instantly turned the heater lever full and switched on the blower. Shut the engine off, coasted to a stop. A roadside investigation of the engine compartment revealed that the upper hose was warm/hot (normal) and the lower hose warm/cool, indicating that the thermostat appeared to be open and functioning proper. Back inside I switched on the ignition and headlights to check the temp. At this point I realized that while leaving the ignition on (engine off), the temp gauge would move from the middle (normal operating range), to the section just under the red zone simply by switching the headlights on or off. The movement of the indicator needle was slow, not instantaneous as if the unit might be grounding completely. Any ideas on this one?

Thanks a Ton!