4RunnerPete

OK gentlemen-
I recently purchased a '93 4runner SR5, 3VZE/340H, 160k mi, once rebuilt. Its been a great truck apart from the small bumps associated with a 100k+ vehicle.
This forum has been such a help for me so far in diagnosing problems and finding the parts necessary to fix them. I need your brains once again, and was hoping you could check my logic here.

Conditions:
-Intermittently wont engage/slips out of overdrive. All other gears shift perfectly.
-No CEL or Flashing O/D light
-Lately engine runs at 1/4 to 1/3 warm on the temp guage-seems to parallel when the O/D gear problem appears.
-PO filled the cooling system with water from his septic tank or something as cooling journals in the block are severely corroded and cooling system has been replaced. (i know i should have checked this before buying the truck).
-Throttle cable to the transmission is tight and in good condition.
-Coolant hose recently popped off of the back of the motor, from what i now believe to be the Engine Coolant Temperature sensor. Fixed, refilled, everything was fine until now.

Logical deduction:
The corrosion in the motor has clogged/blocked/fubared the ECT sensor, causing the truck to run falsely cool and therefore resist shifting into overdrive. The problem is intermittent which leads me to believe that there is a lot of crud floating around the hoses, periodically blocking the sensor from performing its duties. Last night and this morning, the truck was warming up completely, and it would shift correctly.

I plan on jumping the connector terminals on the Data Link port to verify that there is no code for a blown solenoid. According to the FSM (http://personal.utulsa.edu/~nathan-b...30generalt.pdf) another possible issue is a bad valve body. Is there any way to test this aside from dropping it out of the trans completely?

Any thoughts, suggestions, advice would be greatly appreciated.