2002 Buick rendezvous gas gage problems

Hi, I'm new here so I hope I'm doing this right. If not feel free to let me know.
I recently bought a used 2002 Buick rendezvous, last week. Great deal I thought, but found out the gas gage is messed up. It stays at full while being driven, it does go back down when turned off. Sometimes when I first start it it will show what I assume is the correct level, but it jumps up to full when I put it in drive. When the gas level goes lower but not empty (I keep track of miles driven and gallons used), it says the tank is empty until I put more gas in it.
I did contact the seller and they DID say that the gas tank was replaced with a junk yard tank, that they aren't sure if it's the exact one for my car.
I'm getting a diagnostic on it tomorrow to see why the service light and traction service lights keep popping on, so I'm hoping they'll figure this out too. In the meantime I'm hoping to find someone with a similar problem who found a solution.
Thanks.

As far as the fuel gage, there are three possibilities....cluster, fuel sender, or wiring/connection.....

Rare, that it is the cluster.....most obvious is the sender......but there is a connection that goes through the body, just rear of the driver's seat, and comes out the bottom of the body......there are two harnesses that connect to this block....one goes to the fuel tank and the other has ABS connections to the rear wheel speed sensors....

If you disconnect these two harnesses, look for green corrosion in the harness connectors and also in the pins of the block......if present, attempt to clean, and cover with dielectric grease.....in some cases, they are so bad they have to be replaced....if there is no corrosion, the likely culprit is the fuel sender in the gas tank.....

Make sure they tell you, the exact number of the codes.....they will look like P0XXX, for example, P0461, this has to do with the fuel sender circuit.........if there are ABS codes, they will be C0XXX

Thanks man.
Update.:
Mechanics looked at it. Looks like previous owners had cut the original wiring to the gas tank and added new ones to the "new" gas tank. Also, the dash, radio, heater, car lights, and locks go nuts every so often. Everything else looks great accor to the mechanic (under the car).