The censors within the tank are bad, or the fuel pump censor has gone bad. These are relatively easy easy to change. Rarely the dash assembly itself will be bad.
The guauge on my Harley drove me crazy for along time, all that was wrong was the float was getting stuck. Just had to pull sending unit out of tank and tweak it just a tad so it had free movement and wasn't dragging on anything
Well there good possible be an issue with either the gauge itself or the sending unit. The first thing to check is the ground, then try replacing the sending unit.
