to NICO, Steve! We had a problem with the fuel gauge on our old '05 G35, so I suspect that this problem isn't as rare as we would hope.
Ours would show full after a fill up, but only go down to roughly half a tank and stop. There is a cluster self test that you can perform yourself. Check your factory service manual (available at www.infinitiservicemanual.com
) for the self test-procedure. It's located at MWI-35. If your fuel gauge fails the self-test, then I would feel comfortable saying that it's the cluster. Our solution was to just reset the trip meter at every fill-up, and fill up before we got to 300 miles, but the failure happened after we had put the G in the "I don't want to spend any money on this car" category.
There are many companies that repair clusters, but I've never used one, so I don't have any personal experience. I googled "instrument cluster repair" and found several. If I were in your situation, I'd call a few and see if you find one that sounds like they know how to fix it... this one looked promising when I read it http://www.instrumentclusters.com/
There are also several on ebay.