Fuel sending unit, gas gauge problems

The sending unit is in the tank. The problem can be in the gauge, the wiring such as ground or in the sender itself.

Here is something really stange. I had the same issues and found out it was the battery. Yes, the car starter turned over fine and the car would start fine. Changed the battery and the gas gauge quit acting like it was possessed. Dont replace your battery but have an auto parts store test it. Other wise I would go with Paul's suggestion.

also under the driverside sliding door there is what the dealer calls a passthru plug the fuel pump and sending unit wires go thru this plug sometimes g.m. 's all weather plug boots will not keep water out and it gets corroded up. the sending unit is made on the fuelpump as a complete unit it is in the tank and you have to drop the tank to access it. could be a loose wire on pump unit that goes to the sending unit. when the fuelpump it put in the tank it has to be collapsed about three inches and locked into place.the fuelpumps for all silouettes ,ventures and montanas are made in mexico and are cheap made almost all plastic. usually when the gauge starts acting crazy it won't be long until the fuelpump quits pumping. sometimes the pump will still run but won't be pumping the 40lbs pressure that it is supposed to pump. these pumps cost around $300.00 for a replacement pump