KA24DE overheating, can you figure it out?

[OverBoost]

First of all, yes I have searched, but I have a unique situation. I have a '91 KA24DE, completely stock except for a Mishimoto aluminum radiator. The engine has just been rebuilt, every part of the cooling system is new: waterpump, 170 degree t-stat, temperature sensor, hoses, rad cap, circuit sports aluminum overflow tank. I am running Dual Altima fans at high speed on a temp controlled relay.

Problem: the engine keeps overheating after the car warms up and drives around for about 10 minutes. The outside temperature doesn't matter, even though I live in South Carolina. When it overheats, the temp needle gets almost all the way to the top and the coolant overflow bubbles over. The upper hose will get VERY hot (as in burning to the touch) and the hose visually expands (due to the pressure), but the lower hose remains cool to the touch, including the t-stat housing.

I have bled the system numerous times, jacked up the front, there are no air bubbles in the system.

My theory: the aluminum radiator is doing such a good job at cooling, that the t-stat wont open because the incoming water is below 170 degrees.

Can anyone add their thoughts or theories as to why my car keeps overheating? I want some of your input before I go out and spend $70 on a Nismo 144 degree t-stat.

Thanks guys!