I have the same problem (intermittently jumpy when under 1/2 full) and I've noticed if I give my dashboard a #3 BST (British Standard Thump, "fairly solid") the fuel gauge will jump around, and sometimes settle down. It looks very much like a wobble-fault (where some electrical connection is loose / flaky), especially considering the fuel sensor signal is likely to be much higher impedance than the speedo / etc.

I plan to take the instrument panel out, check the solder joints around the fuel gauge, and clean up the ground connector, then see what happens.

On another note, resistors are unlikely to perish over time like capacitors - if they seem to be troublesome, it's often the solder joints - worth going over them if you've got it out and on the workbench anyway. Fixed an apparently dead cruise control unit that way a while ago...