Fuel Pressure Fluctuations

Hi!  

Looking for ideas.  85 hours into a new 915 Rotax, the fuel pressure has begun to fluctuate up to a whole psi while still running within 42 to 46 psi on either fuel pump, BUT only when the fuel selector is on the left main.  Somehow, the fuel pressure remains stable when switching over to the right main (low wing Sling Tsi). The fuel filter and the gascolator filter were replaced.  The pressure regulator is clean of any debris. There are no apparent kinks in fuel lines.   Both pumps push slightly better than .5 gallons a minute, falling within specs.  When switching back to the left main, it re-stabilizes, and the fluctuation takes a few minutes to begin again, starting from 1psi going up over time. 

Does anyone have thoughts on this, or have they resolved a similar situation?

Thank you for any help!