I think that Rollin Thunder has hit upon the problem. I assume that the pump is above water level, if so the air entering the system will allow the water to drain back into the pool. There is a chance that it's the lid on the Northstar - do you see a gush of water coming out of the lid when the pump turns off? It is possible that the lid isn't fully sealed with the pump off (they only fully seal when the pump is running [as an aside - if you have trouble removing the lid, try loosening it when the pump is running :wink: ] ) If you are unable to find any other source, you can install a 'check valve' (a one way valve that lets water only flow into the pump and closes when the pump is off to preserve the prime) - if you go this route, be sure to get one with a union, so you can still blow out the lines for the winter.
That depends on what you mean when you say "gushes out water". The filter is presurized while the pump is running, so some outflow should be expected when the pump goes off. If all of the air is vented from the filter this will be minimized, but will still happen at least a little. All of which is seperate from losing prime.