When you checked the pump for voltage did you back probe the connector? If you just unplugged the connector from the pump and tested to see if there was voltage on that side of the connector you might still not have power going to the other side. It may be possible that you have a bad connector or one that has become corroded which would cause a voltage drop. You may also want to take the pump out and check the filter (I think on our generation its attached to the pump, correct me if I am wrong) maybe it is just clogged and you could clean it or while you have the pump out just power the pump directly, then you'll know for sure whether it really is the pump. Just try checking as much as you are able to so you don't have to keep throwing parts at it. Check everything, all connectors, pins, wires, grounds, and loads.
Hope this helps and I hope I didn't steer you in the wrong direction.