Hello,
Most likely this is the sending unit which you would have to remove your gas tank to replace. It's actually a common problem with these cars.
I found a good thread regarding this issue on some forums located here:
http://www.6mt.net/forum/g35-coupe/31981-g35-fuel-gauge-issues.html
I hope this helps at least guide you in the right direction and save you some money at the same time.
I wish I could be of more assistance, but most likely it's the sending unit like I had mentioned above which would need replaced if it is indeed a faulty sending unit that's causing the issue (From research I've done and having a friend with the same car and same problem, it IS most likely the sending unit, but there are other things that could be causing it as well).
I hope this helps! Best of luck to you and your G35! Beautiful cars! It's a shame for them to have such an annoying issue. It may actually be a factory recall or covered by some type of factory warranty. You may want to call a dealer and inquire a bit.
Take care,
James Trageser
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