Gas Gauge (or Tank) Issue

I have a 2004 Nissan 350Z. During the first few years of the car, I always had a gas gauge that read correctly. Then, I had a recall notice about the gas tank. I took it in and got the part fixed. Now, my gas gauge does not read correctly. It rarely shows that it is full. In fact it looks like the needle “floats” up and down when I have driven it and used some of the gas. Another thing I noticed is if the tank reads below 1/2 a tank and I go in to fill the tank, then start the car up, it looks like the needle just “sticks” and then jumps to almost full. I have put in some gas cleaners to see if that would help and it does a little but two things I would like to know:

If I decide to get this fixed, what is it that I should call this problem? When I mention it to the mechanics, they always act like I am nuts or they have never heard of this problem.

How much does it cost to get this fixed?

They messed up your sending unit in the gas tank. Usually these are attached to the fuel pump. It has a float like a minature toilet tank that goes up and down with the level in the tank. My Riviera sending unit was $300 for the part but the part cost will vary and plus the labor to pull the tank again and/or access the pump mechanism. If it happened from the recall, the dealer should take care of it if the problem involved the fuel pump.