[Dale_K]

I'm not an expert on your problem but I've been thinking about it and have one possible answer. Porsches and many other cars have a warm up mode that tries to make the engine warm up faster to reduce emissions. In other brands of cars this mode locks out high gear but I don't know if Porsche does it that way. This theory would account for shifting in all gears but high. Maybe it could be triggered by a failure of a engine temperature sensor. I had a Chrysler Sebring that would lock out high gear in very cold conditions until the motor warmed up.

I found an item in a Porsche technical document that discusses it.....

Warm-up Map
Apart from the 5 shift characteristics, the control unit also
has a warm-up map which becomes active at engine
temperatures < 90° F. (32° C.). In the warm-up map, the
shift-up points are offset to higher engine speeds, the
transmission starts in 1st gear, and the convertor lockup
clutch is opened. These measures result in both the
engine and catalytic convertor rapidly reaching operating
temperature.

Another place in the same document talks about the factors that go into determining when the car will shift gears. A number of sensors are used but I don't see how a problem with them would apply to high gear only.

Driving In Selector-lever Position “D”
If the selector lever is moved to “D”, an extremely
intelligent driving program is performed.
Shift characteristics adaptation is carried out on the basis
of the following information:
l Accelerator pedal position - Throttle potentiometer
l Vehicle speed - ABS sensors
l Longitudinal vehicle accel. - ABS sensors
l Transversal vehicle accel. - ABS sensors
l Engine speed - Speed sensor/flywheel

Also as I read the document it looks like the transmission controller, which seems to be a separate computer unit that manages the transmission, could have a fault that would generate similar inappropriate actions in the old and replacement transmissions. The control unit sends signals about how to operate the solenoid valves and system pressures. Again, this is just a guess on my part. Maybe a replacement transmission controller would fix it.