when u say it goes up slowly... does it seem as if the car is running really rich when u hit the gas? the only thing i could possibly link to it after gettin hot possibly stuffed the coolant temp sensor. ive only ever really seen one go and that was on a ford falcon. same symptoms. when cold, system is rich. rich is justified for cold motor. but once hot the extra fuel will cause it to overfuel etc. anyways its a good start i guess.