Transmission would not upshift

[230/8]

This is the car:1972 350SL 4.5, early USA model. 142,000 total miles, 3-speed automatic with approximarely 50,000 miles on the transmission which was replaced with a rebuilt by the dealer. All service by the book and last transmission service done approximately 3,500 miles ago. Usually runs fine (but I wish it would shift a bit more firm, I like my old Mustang with the shift kit...)

Here is the problem: Yesterday, Sunday, a warm day, about 85F, long pull, about 15 miles, all uphill, on a slow and winding scenic mountain highway. Engine temp not even close to the red zone. Used the AC a lot and spent most of the uphill climb in low and second gear. No signs of overheating and the car has run hotter on other days. Crested the summit and started down the back side of the mountains, transmission would not shift up from low. Nothing, nada, stuck in low. Got to a turn out and stopped to let the wife go pee. Checked the trans oil. All was fine, proper level, a bit over the top line because it was hotter than normal, but nice clean red fluid with no signs of distress under the hood.

Allowed the car to rest a few minutes with the hood up then took off down the hill hoping to make the service area about 15 miles further on. Presto, no further problems, transmission shifted properly and has ever since.

Strange part: I drove the same road Saturday, a hotter day but a windy day with absolutely no problems at all, SL drove like a peach, shifted perfectly and gave great mileage.

Any ideas why the trans would not upshift? Any ideas why it would be self-healing? It runs fine today and doesn't miss a beat...upshift, downshift, you name it...

Do I have a transmission problem, or just a cranky car with a strange sense of humor? Do these transmissions have a temperature sensor that affects shifting?

Thanks in advance,

230/8