Is the engine starving for fuel? Vapor locking? I have the same pump on my 440, 67 B-body with stock 5/16th line, feeds both the 750 Holley and the 160 shot of nitrous, and it's been 11.24-122. If the car doesn't fall on it's face, I wouldn't sweat the pressure gauge moving around a bit.
"Livin' in a powder keg and givin' off sparks"
4 Street cars, 5 Race engines