Weird overheating issues

So I just replaced the head gasket on my 92 3.0 v6...after test driving it when done, I noticed a weird overheating issue.
When running around town (30mph), the truck runs just fine, oil pressure at 1/2, temp gauge at 1/2.
After a couple of minutes of hitting highway speeds, 55-70, I can watch the oil pressure drop as the temp rises...if I continue to drive at those speeds the temp will continue to climb, and the oil pressue drops until I almost hit the red area of the temp gauge. I drop the speed down to 50mph and can immediately see the temp gauge stop climbing...if I continue driving at lower speeds, 30-40 mph, the temp starts to come down as fast as it went up...the oil pressure starts to return to normal as well.

So I cant figure out what is going on...Im almost sure the thermostat and the water pump only have 3k on them And I never have to stop driving the truck to get it to cool off, just slow it down, and it seems to cool well enough to drive at slow speeds
all of this takes place over the course of 15-20 minutes, not 5 like I would assume if the cooling system werent working at all.


It does not matter if the heater is running at all, I tried the last test run with the temp set all the way to the left (blue) and no fan at all...
The oil looks good, no traces of coolant that I can tell, but it has only been about 40 miles since the HG swap...


Anybody got any ideas?

Thanks in advance for your help.

Have you checked your radiator for cold spots? What weight oil are you using?

I think Im using 5w-30 currently, perhaps 10w-30 but nothing heavier than that.

How would one go about checking for cold spots?

Get vehicle up to operating temp, just to where the thermostat opens. Shut off vehicle and feel the radiator with your hand. Cold spots are indicative of clogs. A restricted radiator can not cool the motor in high demand operations such as driving at freeway speed.
Try a thicker oil (20W-50) to see if the oil pressure stays up.
How old is the water pump? The vanes on water pumps can wear out restricting the flow of coolant.

my friend had the same problem with his 92 and it was the grate v6 head gaskget problem, after he replaced that it ran just fine

One of two possibilites.....you might need to burp
your cooling system, possible air traped in engine.
Or your clutch fan not turning up to speed,
you should not be able to stop it with your hand
when running at temp.
I just spent $900.00 to learn all this.....