I have a 2008 F450 6.4 4 wheel drive dually. (I do not pull any trailers) The problem I am having is that I will be driving and all of a sudden in 1 second my Temp gauge will max out like it is overheating and the dash will tell me that i am running on reduce engine power (Which I do not feel any difference in power). I will pull over and kill the engine and raise the hood and it is not overheating. And now I just pull over and kill the engine and restart engine and it is back to normal. I have changed the following trying to fix it.
1. Fan Clutch Motorcraft
2. Temp Sensor (Motorcraft)
3. Thermostats (Motorcraft)
4. Temp Sensor Pig Tail Connector
5. Water Pump (Motorcraft)
6. Radiator
7. All Radiator Hoses and heater hoses.

Also is there a way on a 6.4 to add a manual switch to kick in the Fan Clutch on high manually. Like they do on a 6.0 called the blue wire mod. the wires are different on a 6.4.

 

Sounds to me like a wiring issue. If your temp gauge spikes that quickly that's almost always a wiring issue, since it is a digital gauge (as in it gets its info from the computer, NOT directly from the sensor). It likely is due to the age of the vehicle (it is 16 years old, after all) and a wire has come loose or worn through or something like that. And it could be anywhere between the gauge cluster and the temp sensor.

My question would be, though, if you don't pull any trailers why do you have an F450 that is purpose built to pull heavy trailers? I'd swap it out for something else unless you've got a plan to go back to pulling trailers.

 

If I'm not mistaken, the instrument cluster receives ECT readings from the PCM via data lines. If you have a scan tool that will read live data, you can check to see if the live data pid for coolant temp differs from what the gauge is indicating. If so, you likely have an instrument cluster issue, especially if that is the only gauge that is failing.