Gas Guage Question: Needle Bounces...easy fix?

[TIMSPEED]

Do you guys know what I'm talking about...my gas guage is functional...when the truck is 100% at rest. When I'm driving, the needle bounces around like it's drunk! But when I'm like at a red light or something, it'll sit on the right spot. I've been told it's the sending unit, but my dad told me that it's just like a little chain that holds the sender thingy on...is broken, so the sending unit is correct when the fuel is still, but when the fuel sloshes, so does the sending unit thingy. Any ideas for an easy fix guys? Thanks!