but the same thing (shifter) happened to my old Z31. Turned out to be transmission issues. I never diagnosed it further, though. I was told that I would need a new tranny, then I got pissed and junked the car (and got the TT shortly thereafter...).
If it was working flawlessly before, I would tend to think it's just an issue with the new shifter. How does the plate under the boot look?