I had a similar problem and after much consternation I discovered an air leak in the suction side. Seems that when the pump was drawing a vacuum one fitting was allowing a minute amount of air to enter and eventually stop the pump but when the pump was off it would reseal itself and not leak. It was a bugger to find and I had actually begun replacing the line from tank to pump when I found it.
Been flawless since and we winterize here in MI too.
BUT NOT NO MORE!!!! YEEE HAAAA
Greg