Hello Holly blue
Also in addition to the heat issue possibility, this is the reason that regulators are so expensive that work well, meaning repeating there set pressures. The fuel pump has a pressure regulating spring in it and could also be causing your fluctuation in pressure. The holley blue comes with a regulator thrown in the deal when you buy them from some sources, this may give you some idea of it's relative value and reliability. That pump will not ever out run your #8 line. More expensive pumps have closer machine clearances and perform more smoothly and reliably as far as pressure consistency is concerned. Running less than 7 psi is recommended in most all gasoline applications. If the pressure drops more than 1.5 PSI from your set pressure the fuel level in the bowl will fluctuate too much give constant fuel ratios and needs to be corrected. Your complaint is not new or unique, but if you minimize the pressure fluctuations your car will perform more consistently and probably last longer too.
Bruce