Gerard,
<<Has anyone else noticed fuel pressures dropping to minimum during the cruise and then rising again on the 320>>
I have observed similar behavior on my O-360.  It was quite alarming the first time I noticed on a long cross country several years ago - every so often, a slow decline from 6 psi down to 2 psi. and then back up again (note: minimum for the engine is 0.5 psi).  It happens only at high altitude.  
I did a little research on the matter.  It turns out Piper did some investigating as this was a common occurrence on the O-360 in their planes.  After many theories were tested and ruled out, they were left stumped by this phenomenon.  It only occurred at low fuel flows, either part throttle or full throttle up high.  They finally left the matter as an unexplained, but harmless, quirk.

Chris Zavatson
N91CZ
360 std
www.N91CZ.com