Here is the story. I bought a G35 from my mechanic friend and it has had an issue with how it shifts. it'll ride out 1st gear all the way until 4600 or 5k RPM and very light throttle input and after that it'll shift just fine.

Today I have: drained the tranny fluid and replaced the filter and gasket, after doing so the car slips in reverse and will still not shift out of 1st gear properly.

The story of what he did was switching out the same VQ engine but from an older G35, and he has a ground running from the transmission to the battery negative terminal. We believe the car needs a new transmission, does anyone have an idea what the issue could be?