I just experience this yesterday, usually i pump shell, then i decided to try out esso(mobil) pump to full myself as usual, normally the fuel gauge will hit the full mark, but it only show in between 3/4 and full.
So when i reach home, off and on the car again, i jump to full when i travel near 40 km

Decided to leave it for the night, so today start car go work, still showing the full mark so tot its ok.
Drove to work, drop to in between 3/4 and full again, travelled 50 over km, so tot heng ah back to normal,
so decided to try again, off and on the car again, went up to full mark again, arghhhh

what is wrong?? anyone who has solved the fuel gauge problem outside other than TCM??
Pls let me know the place and location?? Thank You..