[Kracka]

Replace the thermostat again with a quality Mitsu one, a lot of the time the aftermarket ones are pretty crappy and wear out quickly. I've encountered a few that were junk right out of the box. Have you tested your coolant to make sure its near a 50/50 mix? Also wouldn't be a bad idea to hook your car up to a Datalogger so you can get an exact coolant temp reading, maybe your temp gauge sensor is just faulty.