First off it is good to see a pilot paying attention to what the engine gauges are reading. Too many pilots just blunder along never looking at any engine gauge except for maybe, briefly, at the runup.
The first thing I do when a mechanical gauge is doing something weird is tap the face of the instrument. There have been numerous instances where I have seen a good rap makes the gauge stand up and pay attention
Assuming this is not the problem then I would look at the fuel filter to see if there was any crud in it and finally you might have to change out the fuel sender assembly and see if the problem persists.