I have an 05' Buick Rendezvous with an erratic temperature gauge when engine is hot and idle. When stopped in traffic jams, long stoplights or just moving very slowly for a long period the temp gauge shoots up very quickly creeps towards overheating then drops suddenly as a fan kicks in. Radiator fluid is full. I turn the heat on full blast to blow hot air off engine. Took it to the Buick dealer, they couldn't find anything and welcomed me back if the problem came back. The problem is back, more frequent and happens in either hot or cold weather. Advice would be much appreciated! Thank you!