1999 Pontiac Bonneville Either transmission or electronics

My 99 bonneville runs great normally but at low speeds when the transmission shifts in and out of 1st/2nd/3rd gear multiple times in short succession i.E. Rush hour traffic, bad things happen. The transmission will shift hard and acceleration is hesitant or non-uniform in the higher gear. Once I let off the gas completely and ease on the pedal the acceleration is OK till the next shift. Basically I pretend like its a manual and try to time up letting off the gas at the shifts at 15mph and 25mph. Then after my car sits overnight the problem goes away.

My mechanic replaced the ECU, ICM, manually checked the voltage levels (he had it for 2-3 weeks), and flushed the transmission fluid. After this did not fix the problem he told me that the transmission would likely need to be rebuilt eventually. After researching the issue I want to replace the crankshaft position sensor
in any attempt to avoid rebuilding the tranny. Any knowledge you can share would be an enormous help as I am pretty much on my own to solve the problem.