Had a small issue on my last flight that I can not explain - anyone care to venture a guess what was going on?

I was hmmmm-ing along at 7500' on friday about 60 minutes into a 2 hour flight when my EIS low fuel pressure alarm went off and fuel pressure had dropped to 29 psi (alarm set at 30 psi) and the engine went a bit lean.  I switched on my backup pump and fuel pressure instantly went back to the normal 38 psi.  Flew like that for a couple minutes and then decided to do I bit of investigation so I turn the backup pump back off but the fuel pressure remain at the normal 38 psi for the rest of the flight.  No other problems other than I seem to be running a bit leaner  than normal for  a 12 o-clock setting on the EC-2 mixture control.

My install is pretty much a clone of Tracy's pre-Renisis fuel system other than I've got a high wing (Glastar). Feed and return from left tank, fuel line y after a fuel shutoff valve, inline fuel filters upstream of  parallel  Federal  Mogul pumps  mounted on cockpit  side of firewall, using stock 89 RX-7 pressure regulator, 88 NA Injectors, no damper. I've been try to come with a explanation that would account for such a issue and other than the fuel pressure regulator may have got stuck and the extra flow from turning on the second pump unstuck it.

Perry Casson