Temperature gauge dropping to almost cold at highway speeds

Joe,

thanks for sticking with me through this odyssey. Accessing the ECT sensor on this truck is a herculean task, since the coolant outlet is on the back of the head and the PO destroyed the fiberglass pad on the firewall. I'm reasonably certain that the sensor is fine, because when the coolant temperature gauge drops to low there's barely any heat in the cab and the inlet hose to the radiator is cool enough to grip it.

I followed my hunch and unplugged the MAP connector to the thermostat and took it for a drive on which I'd expect the temperature gauge to dip, 75+ mph on the highway, off the highway for some city driving, and back on the highway. Aside from the expected CEL, the truck was fine and temperature stayed essentially constant. I think either the t-stat is a piece of junk or for some reason the PCM is overstimulating the resistor in the thermostat and causing it to stay open for too long.

Normally I'd go with OEM as well, but I balked at the price of this t-stat; a Motorcraft one is like $150.00. I'm going to go browse an LKQ and see if I can find an OEM for cheap and if that fixes my problem, I'll shell out the money for a new OEM one.