The governor in the trans varies the pressure and is directly related to the speed of the car.
I plumbed a cooler line from the governor port up to the back of the intake where I mounted two adjustable pressure switches from Nitrous Express.
The 42re / A500 also have a "Variable Force Solenoid" that is intended to control the pressure. Are you not using that? Does the transmission function purely on hydraulic control for the 1-2 and 2-3 shifts?