I don't think it's your tranny, judging by the video.
It seems to that the rpm when you shift into 3rd would be lower if 2nd gear were slipping. You'd go through 2nd in less time and shift to 3rd at a lower mph, so the rpm would be lower at the start of 3rd gear. I don't see that in the two videos. Maybe your high stall convertor might cover that up to an extent though?
I think something else caused the higher ET. Wind?, air quality?, heat soaked engine?,
??
JMO,
paulie