I have a 96 f250 with a 460, ever since I bought it (2015), it has had this problem.
It runs like normal until it reaches operating temperature, then a few minutes after that, the temp gauge climbs to around the letter A or L (on my truck it has the word NORMAL spelled out along the temp range of the gauge) and from there bounces around, never making it past L. A few minutes later, it will drop back down to N and allow the engine to idle normally, however, minutes afterwards, it will jump back up and repeat until I shut the truck off. I have replaced the thermostat, I dumped all of the coolant, nothing unusual was in it, it never leaks coolant, and does not burn it in the engine, and heating works fine at all times. Yet whenever it is hot there is air in the top radiator hose to the thermostat. When I shut the engine off, you can hear the coolant bubbling back up to that hose, filling it. Whenever I drive it, coolant overflows out of the reservoir, then sucks it back into the radiator, leaving a practically empty reservoir (I have not pinpointed when this happens, but I assume it is when the temp starts bouncing around). I had my dad (mechanic for 30-40 years) look at it, his employer (when he was a teen) look at it (mechanic for 50-60 years) AND a local mechanic shop. NOBODY HAS FIGURED OUT HOW TO FIX IT!!
All I can think of is that air is somehow getting into the system.
So here I am, hoping you could help me. Please.

 

I have seen this before make sure the steel plate is between the water pump and the timing cover.

 

Ok, I will check that. I will let you know if it is already there or not. Thanks for the input.

 

Yep, I looked and sure enough, its not there. Will be getting one asap.

 

So much for that 100 years of experience. The internet wins again! :-)

 

Yea, my dad and his past employer kinda did that thing that involves the palm of your hand smacking your forehead...
FACEPLAM! that's what I was looking for! They both said they have encountered this a good number of times. So much experience they can't remember it all!

 

Start with a psi checker. Pump it up to about 15 psi and see if it leaks off. Any rad shop will let you use there tester out in the parking lot.