Try running a temporary separate ground wire from the sender to the panel grounds/tree and not locally at the wing root. A lot of gauge/sender problems come from grounding issues. Cheap and easy enough to help diagnose the problem. Just my 2 cents....
Gary Palinkas - Gman.... VAF #161
RV-6 "Sassy" Flying 400 hrs since Oct 2011
Lycoming 0-360 A1A, FP Sensenich Prop
SARL #19 .... Van's Calendar March 2015
Although exempt several ways, =VAF= Dues paid to support this awesome site/family