I have a 2003 Expedition with the 5.4L engine. During normal driving and at normal operating temperatures, the needle on the temperature gauge sits approximately in the middle. On a recent trip to the mountains, after driving at interstate speeds around 70mph I watched the needle on the temp gauge go from its normal position to the extreme hot (needle pegged) in approximately 2 seconds. Once the needle is pegged, an audible alarm sounds and the engine goes into limp mode, with a message telling you this in the message center. After pulling off the highway, there was absolutely no indication that the engine really is overheating. After switching the ignition off for approximately 1-2 min. and restarting the engine, the gauge goes back to its normal position and I can drive the vehicle again until the same thing happens. On a 2 hour trip, this can happen 3-4 times. I took it to my mechanic and after checking that the gauge was fine, he replaced the temperature sensor located in the block. This requires the removal of the intake manifold and is quite expensive. Assuming that the problem was fixed, I took it on another 3 hour trip and it was fine. However, on the return trip the same problem started again. I took it back to the mechanic and this time they cleaned out the radiator exterior of all dirt, grime, bugs, etc. and flushed the radiator. This had no effect and the same problem remains. I'm wondering if anyone else has any ideas. I'm starting to think perhaps the wiring to the sensor is bad, but the mechanic says he checked and everything looks fine. I know it's not beyond the realm to think the new sensor is bad, but since the problem is basically identical as with the old sensor, I have problems believing that. I do not think it is actually overheating since there is no visible sign of this (steam, overflowing fluid, etc.) and because I can continue to drive the vehicle without any problem immediately afterward and it doesn't overheat quickly. Any help would be appreciated.