New to the forums and looking for advice

Situation:

Fuel Gauge plays when driving down the road, plain and simple. I swapped it with a different one and still same result. Fuel tank has been changed to a brand new one and fuel sending unit also. Fuel sending unit has been changed twice. First time by Honda then 2nd and is currently in there, from a junkyard. When the honda piece was in there, I would have a full tank and driving down the road the needle would go from full to half way, and then play allover the place. Junkyard piece was put in place of the honda piece and works better but does not show the last 2-3 gallons of gas in the car. Fuel filter has been changed also, what is wrong with it? any advice is greatly appreciated!