what is causing my fuel gauge to go back and fourth to full then empty.

To prevent making the wrong 'guess' ... suggest instrument panel scan test to pinpoint problem!.. Scanner can test gauge operation plus display in live data what the in-tank module readings are so trouble can be correctly identified.

fuel pump the sending unit is on the pump in fuel tank. have same truck mine did same thing only way to fix is to replace fuel pump have a 2000 s-10 4.3 litre. chevy trucks bad for this mine said full when i bought it until i ran out of gas filled it up and read empty lol