Al
What everyone has mentioned sounds reasonable. I believe, if as you say, "over the years" the transmission temp has gradually increased, my thoughts would be, possibly, sediment building up around your transmission cooler, IF and i mean If the cooler is within your engines radiator and positioned internally in the bottom of the radiator, sediment could be contributing to your progressive rise in temperature over the years.
I would consider having the engine radiator flushed to make sure that is not the culprit. It used to be routine in the automobile industry years back to flush radiators.
Just wanted to give you my ideas, if nothing else is working.
Larry Dedrick