overheats uphill and cools down really fast...

OK so it has a aftermarket radiator and fan as well as hydro assist cooler, Tranny cooler and ac in front of the regular radiator.
So it is fine all around its fine with air conditioning g going it runs fine in 90 degree heat...

If I go up a really steep hill that's really long as in the 330 to big bear 600 feet to 7000 feet in a short period of time it overheats......

Anywhere else its great...

Now here's the kicker......

It goes from the red to back to half in 2 minutes...I stop turn it off and let it sit for a minute or two start it up and it drops back to half or so and takes a bit to go back to red on a steep hill...

So turned heater in full blast and it got super hot but ran it to try and eliminate any air pockets...the radiator was full and the reservoir was down....

Oil isn't milky coolant is Toyota brand .....

 

Thermostat or fan.

 

Yes, id say thermostat or coolant temperature sensor.... or fan clutch is going out

 

Take a heat gun to the thermostat housing. Make sure temp send and gauge is reading correct.