I have a 1998 F250 LD 5.4L
The first problem I had was after my truck warmed (about 3 miles) up the gauge would stay right in the middle where it should and blow warm, but when I stop and idle my truck the heat goes cold, (gauge stayed normal). So I replaced the Water Pump & thermostat.
That didn't fix the problem, only made it worse. Now my gauge stays at the Cold for about 5 miles then shoots up to HOT instantly and the check gauge light comes on for about 30 seconds, then falls to normal for about 1-2 min than to cold. Then again it shoots back to hot and then falls to normal and stays there.
I also have burped the air out of the system, and think I have all the air out of the system. I did this by squeezing the top radiator hose while the engine is running and circulating coolant.

Any ideas of where to look next?

Thanks

 

Have you checked the 2 heater hoses going into the firewall? They both should be equally hot, if not the heater core could be clogged.

 

ya I did, they are the same temp. I took it to a friend this weekend, and he is almost positive it's a head gasket. The anitfreeze smells of exhaust and a lot of air bubbles are bubbling when the engine is hot and running.

 

All the signs of a blown headgasket. Did he do a leak down test or pressure test?