My ranger keeps overheating!!!

Hey guys i know its not a mustang but i figured since its a ford you guys may be able to help me out.

Anyway i have a very weird overheating problem with my 2001 Ranger 3.0 liter.
It only happens about one time per one tank of gas, so 95% of the time it runs fine. Whats happens is, it gets to operating temp and stays there for a while, then out of nowhere my heat blows cold. The temp guage will then drop a bit and seconds later it skyrockets up to the point where it overheats. By the time i try and pull over to shut it off, the temp drops back down to normal.

I have replaced the thermostat and radiator cap thinking it might be the problem but no luck. I have also bled the system of air and it has fresh coolant in it. All of the hoses appear to be fine also. Something weird i have noticed also is that the overflow tank does not have a vent, so the whole cooling system is not vented.

I need your guys help to figure this out!!! It has made me late to class a few times and is getting very irratating!!

Thanks!!!

It is not a guage problem because when it reads hot it actually will puke coolant out of the overflow. I do have a vented cap also. I can hear the coolant bubbling up in the overflow, after i shut the truck off from normal driving. And the check guage light will pop on when it gets hot, and then go off when it cools back down so i couldnt do a code scan without the check guage light on right?
Ive heard that it could be an air pocket in the system, but ive bled the system already and it still happens.

Anyways, thanks you guys for the help!