I'm a little confused. You say the planets aligned twice. Are you saying the pressure and flow were fine twice, and then the pressure and flow dropped? If so then that tells me the system is capable of working fine and something is changing dynamically. What are the pressure readings working and not working if you have them? Actual numbers will help determine the amount of change. Does shutting the pump off for a few minutes, then back on cause it to repeat the working, not working cycle? Are there any air bubbles visible at the returns or in the pump basket chamber? I'm kind of thinking small suction side air leak. With that much of a suction side length the suction vacuum at the pump will be higher than normal so maybe this is causing something funny. It's puzzling that you said it ran for a few hours then went south. I don't know what it would show but can you try running without the cartridges just to see what happens? There is an explanation for everything and just a matter of digging to find it.
Al