I've got another idea that might relate to why Cherokees, RV's and your 320 randomly exhibit the same phenomena.  The key element is that for a given plane the behavior is consistent.  I suspect you are reading the pressure with a conventional mechanical gage, like in my old Cherokee, with an "old fashioned" can-type sending unit.  If not, you can hit the delete button now.  Most mechanical gages are vented through miscellaneous leaks in the case - there is no dedicated vent hole that allows the measured pressure to be referenced to atmosphere.  What would happen if the case was inadvertently sealed?  The interior would probably be at approximately seal level pressure, starting at the pressure in the factory in which it was built and then ever-so-gradually assuming the pressure at which the aircraft is stored, possibly taking anywhere from days to months get there.  When you go to altitude the absolute fuel pressure will drop, since the fuel pump produces pressure relative to the existing atmospheric pressure.  But the gage would be(inadvertently) measuring the pressure relative to sea level.  The pressure difference between sea level and 8,000 feet is between 3 and 4 psi (to be precise the standard pressure at 8202 ft is 3.9 psi less than sea level).  That could explain the difference.  And the real (what we call gage pressure) might a little higher at the higher elevation because the flow rate is a little lower.  At the slight risk of damaging the pressure gage you could drill a tiny hole in the housing at the bottom in order to provide a positive vent.  It's probably not a fancy gage that can be disassembled easily, so you almost have to just take a chance.