Had this problem a while back. Didn't fancy the silly money Vx were asking for a replacement fuel gauge sender, so I persevered and eventually found what was up with the one in the car.
If the gauge sender is the problem, there's possibly a completely free fix.
Have a look at the piccy of this one from Ebay:
http://cgi.ebay.co.uk/VAUXHALL-OPEL-OMEGA-FUEL-PETROL-TANK-SENDER-UNIT_W0QQitemZ350224853093QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_SM?hash=item518b072865The metal rod that holds the float has 3 bends in it. One at the float, a smaller one where it approaches the gauge workings, and one where it pivots about as the fuel level changes.
This last bend takes any forces from the float swishing backwards and forwards in the tank, and, over the years, had opened up from its original 90° angle ever so slightly. With this angle bigger than 90°, the pressure of the wiper on the resistance track is reduced to the point where it may lose contact.
If you carefully extract the metal rod from its plastic fastenings etc., then give that critical bend a little 'tweak' to close it back up, you may find that your problem goes away.
