If its not venting (hissing) from the pressure relief cap on the expansion tank and it sounds like the thermostat is opening ok... I'd be tempted to replace the temp gauge sensor... could be the sensor is faulty and reading incorrectly. Not to be confused with the CTS which the ecu uses! With over heating you wouldn't be able to hold the thermostat to rad hose or touch the engine/rocker cover etc because it would be boiling hot!! Although if the coolants never been changed then its never had a cambelt/water pump replacement, that can be anything from 60 -> 100k these days... so its due if it definitely hasn't had one yet!!