Since they are all doing it, I'd look for common sources. I'd check for a bad connection in the positive lead maybe or a corroded/burned wire somewhere. You also may be dealing with a system-wide voltage fluctuation as well, which could be a loose belt on the alternator or the voltage regulator may be on the way out. Is the tach doing anything odd?
But the fuel gauge, it's probably going to keep doing that. Very, very few aftermarket gauges are dampened and the fluctuation you are seeing is the fuel splashing around in the tank.
My oil line fix
My 100k stopped odometer fix
Brace for impact, I'm in a "modifying mood". Massive thread count inbound.