I don't know the exact setup on a 2G N/T, but I had some similar issues in the past.
If it pops out of gear then it sounds like it isn't fully engaged.
What is the condition of the shifter ends that connect to the trans? Maybe the rubber grommets are worn and not forcing it into gear like it should. There is a "skateboard bearing" mod that helps with that.
Are there adjustment nuts on the shifter cables where they connect on the shifter base in the car?
Does the shifter move at all when you give it gas? Has the end cover ever been off of the tranny? Sometimes the end nuts work their way loose. Slop in the gearsets will mess up engagement. Shifter wiggle/movement while in gear?
If you can't get it into gear when the car is off then you might have a clutch disengagement issue. When did you put that clutch in?
With aftermarket PP they sometimes need a little more travel at the slave cylinder to fully disengage.
I also had a clutch completely change the way it disengaged after it was fully broke in. I had to do quite a bit of adjustment to the rod that screws into the pedal assembly.
This is all from a 1G AWD, not sure how much different a 2G N/T is...