Was a bit like that. Far too many man babies around imo.
I know what you are talking about. I change the fluid on my wife van and about 8k miles afterwards it let go. With Saabs not having a trans filter I would say its better to change the fluid at least once a year. Was driving great up until a couple of weeks ago. We had some very hot summer days here which I think added to my troubles. Trans probably over heated. Wonder if there is a way to add an additional trans cooler fan. As long as it is a mild weather day it drives okay. Below 80degrees.I hope that works out. I have heard conflicting things about changing trans fluid. One crowd says not to do it if it has been a long time between changes. It will destroy the trans. The other (the minority) says change it no matter what. The other crowd is full of crap. Me; I just develop leaks and change it a little bit at a time.
If nothing else it's heat being generated in different locations.There's definitely a lot of heat being generated in the torque converter at idle in D. The converter has to slip because the car isnt moving and the engine obviously still is. all of that energy is transferred into the trans fluid as heat.
When moving the converter is locked and very little energy loss occurs.
Idk about heat at speed vs at idle in D, but it seems plausible.