Was the Check tranny light blinking?
I have had that happen 3-4 times. It was the tranny "lockup". A device that locks the torque converter at ~40>45 mph to improve fuel economy.
Every time I had the problem, it was at the same place, on my way home from skiing, cold night, first uphill climb.
It kept blinking for the whole 1 hour ride home every time. Then next day it was OK.
I think that under the cold conditions, the lockup device doesn't move into position. The computer doesn't notice that there is some slip between the engine RPMs and the wheel speed until I hit the hill and the load causes the tranny to slip a little.
Hope this helps.
Petecal