I have a '96 SL with 155K. Sometimes the engine temp guage reads full cold and the engine idles high when I've driven long enough that it should be up to normal temp. Then all of a sudden, it will jump up to normal operating temp. Then it may or may not go cold again. Some mornings it does this, but not every day.
Do you think it is an electrical problem, or a sticky thermostat? The thermostat is the original, 11 years old. The coolant temp sensor on the end of the block was replaced a few years ago.
Larry