I have noticed a problem with my transmission ('61 LeBaron) on a couple of occasions.
In hot weather and after spending a long time idling or crawling in heavy traffic, the transmission becomes slow to change up. It will change eventually but need much higher revs than normal. On these occasions the temp guage reads higher than normal but not anywhere near overheating. Once the car has cooled off, the transmission behaves perfectly again.
I have two theories. One is overheated transmission fluid. The other, less likely, is that the excessive heat under the hood is causing the throttle linkage to expand to a point where it affects the upchange.
Has anyone else had a similar problem? Suggestions would be welcome.
Best wishes,
Tony V.