Are you certain that the car is truly overheating?

A faulty temperature compensation board will sometimes give a false high reading. It doesn't make sense that it would be overheating at highway speeds, unless the coolant isn't flowing.

Removing the temperature compensation board and hard-wiring around it is pretty straightfoward if you're comfortable with using a scredriver to remove the instrument cluster and using a soldering iron to run a wire between two pins. The board is prone to failure and is really unnecessary -- all it does it "smooth out" the temperature reading so it always stays at "normal" rather than fluctuate with the actual temp.

Before I replaced the TCB on my wagon, I thought it was overheating all the time.

I wish I could think of a simple way to determine whether your TCB is at fault or not, but I can't off the top of my head. I suspect that one could somehow measure the signal directly at the sensor, but I'm not sure if it puts out a voltage or a resistance or what normal values would be. Anyone?