When I used the scan gauge to monitor fluid temp -- I had some similar results -- without the big temperature drop. I closely monitored my transmission temps on a big highway trip last year, but can't find my results. But basically - high engine demand (such as long hills) created greater heat and this is transferred to the transmission. The ATF should be at an operating temp between 165-190F -- and from some old threads it begins to breakdown around 225F.
After studying the coolant hose layout -- I'm convinced that the plumbing is wrong (and I'm not the only one who came to that conclusion:
http://forums.vwvortex.com/showthread.php?3604848-EV-01-03-transmission-coolers). In the Eurovan, the ATF cooler/heater is fed off of the hot coolant return line (hot engine coolant enroute to the radiator to be cooled). This is a good thing to bring the ATF up to 165 -- but terrible to keep it there when engine temp gets high.
So if you're having funky temps swings -- I think you need to look at a couple of things -- one is an ATF fluid change (ensuring proper levels), and the other is an inspection of your engine coolant system. Not a bad idea to zero the fluids together. You can also look into adding an external (thermostatic) oil cooler inline with the ATF system. And -- inspect the fan control resistors which are magnet for corrosion. If these are breaking down -- then the whole coolant system is not capable of working properly.