Another one with this issue on my 0-320. Over the years I have tried just about everything. The best solution was installing the FC-10 Fuel Computer
as it has a selectable electronic dampening function which most fuel computers do not have. This makes the flow reading more stable (not perfect) but slower to react.
For my aircraft the actual cause of the fluctatuating reading are changing G forces caused by turbulence. In smooth air the readings are always stable.