I would suspect the fuel level sender, cars of this vintage are known to have this issue. Count your blessings, mine jumps around (sometimes) the rest of the time it reads dead empty. I never know how much fuel I have in the tank of my 73 450SL.

__________________
Gary


1973 450SL
1999 Silverado 4X4 Ext Cab
1999 Mercury Sable
1972 450SL Parts car

Previously owned keepers:
1967 MGCGT - 3L-6
1972 Mustang Fastback - 351C

Current Cylinder Index: 37