So I took my DI do the orv park saturday, and overheated on the motorway on the way there. Eventually nursed her to the park and after letting her cool proceeded to have a fantastic day playing in the dirt, with the engine maintaining a steady 1/3 mark on the tstat. Then she almost overheated again on the motorway heading back (mind I was going about 60mph). Yesterday I drained all the coolant out and sprayed out the radiator with a hose, then refilled with fresh coolant and a bottle of watter wetter. Bled the system and ran it pretty hard to check for leaks, air bubbles, etc. She was then fine all day driving around. At around midnight I got on the motorway and after about 8 miles she overheated AGAIN! 'round this time I'm getting quite frustrated! Sometimes it's über cool and others it's too hot. I've got a bipolar cooling system.

Fan clutch seems okay. tstat doesn't seem sticky (I'm replacing it today just to be safe.) What else could possibly be wrong??

 

What year and mileage?
Have you inspected both the electric fans as well as the mechanical, how about your t/stat.

 

'98 with 129k miles. Only the driver side aux fan seems to engage when the a/c is on. When she overheats sometimes they come on when I turn the engine off and sometimes they don't. There's no rhyme or reason to it. t/stat seems okay, I can feel hot coolant flowing through the lower rad hose.

 

Sounds like the T/stat is not opening all the way.
That would account for the slow speed cooling and the high speed over heating.

 

I too am leaning towards a bad t-stat.
If it was overheating at low speeds then I would say water pump or fan clutch.

 

You have to have both electric fans running along the a good viscus hub and t/stat.

 

Any chance the radiator itself has developed a blockage?

I'm starting with the least expensive thing first, replacing the t/stat tomorrow, and I'll boil the old t/stat to see if it works.

 

Everything is possible, but even more so, you can't run on only one electric fan.

 

I was under the impression that the aux fan was designed to kick in with the a/c or during overheat conditions, otherwise the viscous fan could do the job on it's own.
I was also planning on eventually pulling the viscous fan altogether and replacing with a spal electric main fan, however, research is telling me that at highway speeds the ram air effect is much greater than what any fan could deliver. Since my car only seems to overheat at high speed it's very unlikely that the fan is the culprit...

 

The electric fans run when you turn on the a/c and during a overheat situation, they do not come on otherwise.
So if you turn the a/c off and drive the truck for 6 days non stop they will not come on.
The clutch fan "freewheels" until the engine reaches approx 210*F and then it will come on and cool the engine. once under 200*F then that shuts back off.

If you want more info find my "fan question" thread.

For your situation replace the thermostat first, if the problem continues then flush the cooling system and see what happens.