OK, I've done some research in archives + Googled this "problem."

I ran across this Q&A here

Question: 1990 Toyota Celica 1.6 mileage: 144,000. Hi, I have a 1990 Celica ST. This is my first Toyota and I have a weird problem. When the car is idling or slowly moving the temperature gauge works fine, then when it gets up to highway speed the gauge will slowly inch its way down to zero. When it registers in the cold area of the gauge the car will go out of overdrive and downshift to 3rd gear. In other words, as long as the temperature gauge is working overdrive works. Can anyone help with this problem? Was this a common ailment of the early '90s Celica's? Is this a big repair job?

Answer: Replace the thermostat in the cooling system. Yours is undoubtedly stuck open allowing the cooling system to go below normal operating temperature which causes the transmission to shift out of overdrive. This is not a common problem and it is not a big repair job.

[italicized emphasis added]

Eh. So a car that does not shift in OD when cold is considered normal operation?

Comments welcome.

--
Norm Cook Vancouver BC; 1989 745T 204,000KM