Mitsubishi Repair: engine overheating, replacing a thermostat, engine coolant temperature

Question
2004, Lancer Ralliart, 2.4, 4dr, Auto.

This problem occurred recently during a drive to work in the heavy rain, and has been raining for the past days. The Engine coolant temperature gauge reaches "H" level, and then during a later drive it doesn't. This problem seems to come and go for the past 2 days now. The radiator and reserve tank fluid is at normal level and the fans works. The car has never been in an accident. What might the problem(s) be?

Answer
Ryan,
You described a typical symptom of a stuck thermostat.  When your thermostat sticks, it does not allow your coolant to flow properly throughout your engine and radiator to help cool it down.  Replacing a thermostat is relatively simple and cheap, though you should have your dealership do it if you are still under your warranty.  This way they can also check your engine temperature sensor to be sure that is behaving properly.  All these services are free if still under your warranty.  But my guess would be that it's just a sticky thermostat.
Good luck!