In addition to what theop says, it sounds like you don't have enough water in the overall system. If I read what you said correctly, it sounds like your return pump doesn't have water to pump back into the tank when the drain stops. If this is the case, you'll need to add more water to the system. Hopefully the sump will accomodate more water and still have enough reserve room to handle the overflow water and back-siphoning through the return pump when you turn things off.
Jack