Where did you tap into the fuel system with the line you run to the gauge ? If you are at the inlet banjo for the injection pump and without a snubber or isolater you will receive pulses that will give you the vibration and erratic readings. Is this something that just started to happen or did you just install the gauge and this is typical behavior since you have installed it ? If it read solid before and just started show this signs I would keep a close eye on the pump.
I have gone through more than my fair share of lift pumps and they all started their ultimate failure by first showing erratic FP when under throttle. Hope I helped.
Scott