I am getting a lockup failure code from my TCU, and then lockup is disabled after that. I have done the following:

1. Replaced Transmission with a super low mileage unit from a wreck.
2. Replaced Torque converter with a new one at that time.
3. Replaced solenoids for valve body.

I'm wondering if this isn't some sort of issue with fluid flow through my cooling system or perhaps its my TCU?

What would you folks check next?

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1999 Montero Gen 2.5