5 quick operation and Maintenance Qs

I agree the fuel economy drop is too much. I run 18 mpg pretty much all the time.

Your diff can not "slip", it is just 2 gears (pretty much), but it can wear out. If that were the case you would hear a considerable amount of noise from the back of the car.

The torque converter can slip (and always does to some degree. hence the transmission cooler on automatic cars) causing too much heat. there are transmission temp gauges out there. if the car shifts firm and the RPM are consistently the same at a given speed then probably not your issue. A transmission cooler mounted out of the radiator may help. If you have an auxiliary trans cooler, try removing the lines from the radiator, that would help eliminate the transmission as your heat source.

Yes do look for air flowing around the radiator, interesting that the temp stayed the same under a load at lower speed.

as far as pushing the pedal "to far" you have to remember the car was designed to drive 55 mph, and there is only 1 relatively small air inlet for the engine to breath. Switching from my Brothers 360 to my /6 is a very dramatic change in the go pedal department. I watched you video and it seems to accelerate just fine.