Check the resistance between the tank and the frame. I'll bet you've got a bad ground in there somewhere. Try running a ground wire from the sending unit where it bolts into the tank, to a known good chassis ground. Also check the instrument panel ground to the dash. Anytime turning on another circuit alters the performance of something, there's a grounding problem somewhere.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!