My car did the same thing. Any excessive electrical use (radio on, lights on, a/c on etc) the temp guage would climb. It did this under warranty and after about 5 attempts, the dealer found a faulty ground wire to the battery or chassis and then added a second ground for backup. This fix lasted for over a year. It just started doing it again. I can't find the ground the dealer added and the ground to the battery looks ok. I called the dealer but they want $100 just to look at it. You'd think they'd back up their warranty but no. Not sure what I'm going to do, but it was fixed for over a year.
Specifically, this is what the repair ticket says verbatim. "51226 CHECKED TEMP SENSOR AND FOUND NO ISSUES. DUPLICATED COMPLAINT AND FOUND THAT CHASSIS TO ENGINE GROUND WIRE OVERHEATING. CLEANED CONTACT AREA OF WIRE AND ADDED AN ADDITIONAL GROUND WIRE. CLEANED BATTERY TO ENGINE GROUND. RETESTED AND COULD NOT REDUPLICATE COMPLAINT."
Sunday, August 29th, 2010 AT 4:18 PM