also the senders are junk even when new. it might just be a bad sender even if it is new.
also check the grounds, and that the wire is good and the plug end is good.
If the wire is busted in side the plastic coating it might lose its connection when the wire vibrates at certain RPM's and make contact at others.
1987 black Milano Verde, 1987 (made in 1986)blue $200 gold with ABS and LSD
1972 White spider 2000 Veloce