Is the issue that the level in the sump is changing or is it just that the sender is bouncing around in the tank. If the sender is just bouncing around in the tank I think an electrolytic capacitor from the sender line to ground might help filter out some of the bouncing.
Try a 1000 uF capacitor with a voltage rating of at least 24 volts and see if this helps.
Make sure you get the polarity correct, plus to the sender and minus to ground.
Here's one from Radio Shack:
http://www.radioshack.com/product/in...entPage=family
If it's still moving too much you could try this 4700 uF part:
http://www.radioshack.com/product/in...entPage=family
I have a Rick's tank too but I haven't connected the sender to the fuel gauge yet.
Don