My Pentair Intelliflow VF is programmed to run for 24 hours at 20gpm. After a back wash, the pump will run at 1340 RPM at 3 p.s.i. DE filter pressure. After running for 24 hours, the pump speed creeps up to about 2200 RPM @ 15 p.s.i. filter pressure. After a 15 second backwash, the RPM drops back down to low 1340 RPM and the process starts again. Why doesn't the pump speed stabilize and not continuously creep up?