when u say "a little driving" do u mean, like minutes after u fill 'er up or like a few days when the level in the tank would've gone down a bit?
I had a similar prob w/ my sender, where the float would push the needle on the rheostat upwards when filling up but by the end of the week, it's telling me I have a 1/4 tank to go when I know I should be on half tank
turns out the needle was loose and the float only held onto it when the level was right up...I suspect it's the same way w/ urs
the sender unit should either be under a panel in the trunk or under the backseat...but it's best to carry it by ur electrician BUT with a 1/2 TANK of gas (u should know ur car's gas consumption behaviour enough to guestimate when it's 1/2 tank)
I say only 1/2 tank because the prob only happens when the level goes down so let him see for himself what's going on w/ it...hope u get it fixed soon...
OH! and DON'T let anyone tell u u need a new sender for $X-hundred and to leave de money and come back l8r 2 install...most rheostats will work, once adjusted/calibrated properly..