Is the genny supplying power over the same wires as the house circuit (disconnected obviously!!), or directly?
When the house circuit is trying to start the pump, do the wires get warm? (WARNING! Testing this may be hazardous to health - make sure you grab hold of a bit of wire with good insulation!!)
It's possible you've got a corroded or rough connection from the house to the pump - this, being high resistance, will get toasty hot; but it'll also sap the power, presumably to the point where the pump can't get started.
Without further info, I'd diagnose a rewire...