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There are a few possibilities for what you have going on.
It could be an internal hydraulic issue in the trans but I'm leaning toward electrical at this point.
The first thing that should be done is use a scan tool to see what fault codes are setting in the transmission control module. The problem you are experiencing should be setting a fault code.
Since the valve body lead plate has already been replaced the most likely causes for this (if electrical) are going to be shift solenoids, wiring, the TCM or the electronic shift module (ESM). The ESM is in the shifter and communicate shifter position to the TCM. Most problems I have seen with your symptoms have been due to a bad ESM.