Why would fuel gauge go up and down. on 2005 Buick Rainier

The fuel gauge go's up and down at any giving time but fuel pump is working fine.

1 answer 3 comments
TEST GAUGE AND FUEL TANK SENDING UNIT.MOST LIKELY SENDEDR IN TANK
Thanks is this easy to test and can we do this at home?
IF YOU CAN DROP THE FUEL TANK OR HAVE A SCANNER
Oh. No scanner.. thanks do you know about how much that cost.and one more? Do you know what would make the low beams go out for no apparent reason? High beams and fog fine.