You can check the sender using a test meter, should read 0 ohms when
full and 240 when empty. If you know how much gas you have in the tank,
you can
figure what your ohms should be to determine if the sender is good.
If the sender reads steady, then problem could be with your gauge, it may
need a
rebuild. Does it move back and forth when driving only, could be a
short someplace in the wiring.