Sometimes people put thread tape on the sender unit threads to seal it like other coolant related fittings. The tape does not always allow for a good ground. I have removed the tape and cleaned up the threads on both surfaces and fixed a similar problem.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine