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I too am confident that the PCM is commanding the 1->2 shift by turning off Solenoid A. I cannot think any VB problem that could cause a 1-2> shift without turning off Solenoid A.
Granted, an intermittent Solenoid A or its wiring could cause it, but that would cause random time 1->2 or 4->3 shifts.
Joe, those are the fixed WOT values and I think the OP is asking about partial throttle.
The part-throttle shifts are controlled by a table of values depending upon the throttle position %.
If the TPS is defective and reading low, the trans will shift at very low speeds; unfortunately it will also command too low a line pressure with the result of ruining the clutches.