I had same problem last winter and it was a monovalve. The pump is a usual suspect, but it was working fine. You can feel if the pump motor spinning or not. The monovalve was sticking in closed position and the water pressure at idle was not enough to overcome the friction. At higher RPM the valve would work OK. The new one fixed the problem. You could also try to take it apart and clean armature and inside of the coil. But first check you aux. pump.
Mike