Mitsubishi Repair: random overheating, coolant level, head gasket

Question
I have a 2001 spider eclipse four cylinder. I think it has a mind of its own, I have replaced the thermostat twice, installed a new radiator, flushed it and still it randomly overheats. For two weeks after replacing the thermostat it was fine and yesterday the temp. shot up after only 10 min. of driving. The top hose is hot while the bottom one is cool or just warm, like the thermostat wont open or something. When I drive it tomorrow it will probably be fine until the next day. Running the ac and the heater usually bring the temp down but still most times the needle is just over half way up. We don't think it is the pump, no leaks anywhere but how could we know for sure. Please help I am soo lost.

Answer
Charlynn,
Keep an eye on your coolant level.  It's possible your head gasket is warped; causing a blockage and/or leaking off coolant into the exhaust.  By turning on the heater, coolant passes through the heater core and helps cool it.  

Have your mechanic perform a compression test to verify if your head gasket is blown.  It would also be a good idea to replace your radiator cap, if you have not done so.  If the thermostats used are aftermarket thermostats, you may want to consider replacing it with an OEM one directly from the dealership.  They're not too expensive.  Check your engine oil level and transmission fluid (if you have an automatic transmission) level as well.  Lack of either of those oils can cause your engine to overheat.
Good luck!