Smart Craft fuel gauge acting up

Check your ground wires by where the wiring goes to the sending units on each tank.  Also check the butt connectors on those wires, they do tend to loosen up over time.  I have replaced those connectors on the boat I currently have whenever the sending unit acts up on mine.  Then it goes back to working just fine.  One way to check them is while you are on the water, and it starts acting up is to tug on the wires going to the connection just a bit.  If the gauge goes back to working fine then you have spotted the problem.