This has been a chronic condition for our V8 2001 LS. The engine temperature gauge will swing up to full scale. But, turn off the ignition for a few seconds at a traffic light and as likely as not the gauge will drop to normal for the rest of the trip. I've sort of ruled out a wiring problem because the temperature creeps up over perhaps 20 seconds. If the wiring was intermittently shorting the gauge would most likely jump rather than creep. The computer seems to be "fooled" along with the analog gauge because it doesn't take long for the computer to feather the engine in an attempt to reduce further overheating. Odds are the computer does not control the analog gauge so the abrupt return to a normal reading is unlikely to be an artifact of rebooting the computer when the ignition is cycled. These observations lead to the tentative conclusion that the temperature sensor has problems. Over the years the car has gone to the dealer twice over this issue. Each time the costly bill has detailed thermostat replacement and other ministrations to the cooling system. Each time the problem was "cured" for a year or two. But on only one occasion was the temperature sensor replaced. Yet every time the problem has occurred the symptoms were identical, and in every case "curable" for minutes, hours, days or months by simply turning the ignition off and back on. Have any of you run into this spiking temperature reading problem? What's the best way to test the coolant temperature sensor?