I have a 1992 300e with 116k miles. This morning I fired the car up and the coolant light was on. Checked the coolant level and it was good. The light when off in less than 30 seconds. Then, I drove to work and was sitting in a drive thru and the engine temperature skyrocketed upward (it was 81 degrees outside). I turned on my heat and the engine temp fell pretty quickly.

If my coolant level is good, what other checks can I do to try and find out why my engine temperature is going up like that?