Low Fuel Pressure w/ Backup Pump On

Have you checked your fuel filters recently?
Post a drawing of your fuel plumbing.
Are the electric pumps in series or parallel with the Mechanical pump?
How does the fuel get past the electric pumps when they are off?

The Carbs do not care what the fuel pressure is.
As long as enough fuel arrives on time, all is well.
Apparently, 1.5 psi is enough pressure to keep the carbs well fed.

By the book, the minimum fuel pressure should be .15 Bar (2.2 PSI)

Hard to understand why lowering the demand for fuel should decrease the pressure... could it have something to do with the change in the aircraft's attitude when levelling off?

Bill H may be on to the answer - mechanical pump may be having to overcome a restriction caused by one or both boost pumps or as also suggested, a partially blocked filter. The restriction may be a valve arrangement in the pumps. Once this has been overcome "pressure steadily climbs back up to around 3 psi"

I really appreciate all the responses and they make interesting food for thought.  As far as obstructions or restrictions in the fuel filter or the 2 Facet pumps, I should add that the fuel filter is new per the last condition inspection with only about 10 hrs. or less on it so it's unlikely getting clogged up already but who knows so I'll check it out.  BTW, it's the 100 micron filter that Rotax recommends for the 912 engine.  I also filter my fuel with a funnel and use only non ethanol premium autogas for what that's worth.  

As to restrictions in the Facet pumps which are about 7 years old installed just downstream of each wing tank in series with the engine driven fuel pump, maybe that's the issue but not necessarily because of debris.  I was just at the Facet web page where they were talking about the inverse relationship between fuel flow and fuel pressure, i.e., pressure is higher at lower flow and pressure is lower at higher flow.  I need to check this out further by watching exactly what's happening in the transition from take off to leveling off (and maybe some other scenarios as well) but it could be that at 5000+ rpms, the fuel flow is so high that the pressure is dropping off because the Facet pump somehow can't keep up with what the mechanical pump is doing?  The Facet pump is after all, just a backup pump in this application.  The model of the 2 Facet pumps is 41827 but so far I can't find the specs on it for psi ranges, etc. because it's 7 years old and they evidently don't make it anymore;  fyi, these pumps were installed well before I owned the airplane.

Anyway thanks again for your thoughts and ideas.  Please feel free to comment further.  I'll keep researching in the meantime.

The pressure vs flow thing is just Bernoulli's principle  :)

You say the e-pumps are in series with the mechanical pump but, as Bill had asked, how does the fuel bypass these when they are off?  If there is a bypass with check valve around the e-pump (as the installation manual shows), the mechanical pump should be able to get as much fuel as it needs no matter what the e-pumps are doing.