Maxima '05 Shifting Problem slips.

[Anthonyzitro90]

Since I bought the car ive been having tons of problems, i already replaced the radiator, alternator.
So a month ago I finally bought a used transmission hoping that would make the car run smooth again, but I was wrong. ( at some point the transmission got stuck in 5th a couple times, i had to park turn off the car and turn it on again just like a pc and it would run ok again for some time) Even with the "new" used transmission is got the same problems what can be wrong? The first guy who saw the car told me
It was the valve body kit amd that it was gonna cost me 1600... ( he showed me the codes that appeared on his tablet connected to the car and it said that the valve body kit was the problem).
I figured i could buy a used transmission and the problem would be solved. So i took it to a friend that knows how to replace trannys and he says that the car is almost the same and has no idea why, at this point he asks another friend and tells him the problem is not in the transmission is somewhere else ( but doesnt know himself ... ( this 3rd guy used the code reader from the car and it doesnt show any transmission related problems) but the car runs the same as it used to.. ( slips from time to time, heavy loud shifting. ) what can ve wrong with the car? Anyone that had a similar problem? Any thoughts?