Fuel rail pressure fluctuate between rpm's
 

So my 06 idles at about 6500rpm @7000pis, and when driving down the road at about 2200rpm @ 21000psi, but when I drive from 1900-2100rpm, my fuel rail gauge fluctuates very rapidly, the truck runs fine with no hiccups, I put a solid plug in the relief valve, and changed out the FCA, the truck has 600000km on it, with an air dog, I guess my question is what would be causing the gauge to do this, thanks

Well I guess if nobody replied it can't be a huge deal, which is good