A very common ailment, it seems. This has bitten both the 115 Berlina
I used to have and the Alfetta I currently have. Best diagnosis I can
offer is that the resistive part of the sender in the tank has gone bad.
Typically, this seems to manifest itself as the gauge working fine for,
say, the first 1/4 tank or so, then flickering to empty as the level in
the tank drops a bit more to, say, 1/2 tank, then flip to empty and stay
there until you fill the tank again. The light seems to be controlled
by a separate switch. The arm and float are still operating properly.
Thru the dim cobwebs of time, I'm almost remembering fixing this on my
69 Berlina..