I had the same problem, it turned out to be a blockage in the level sensing tube, a rubber hose from the bottom of the drum to the water level pressure sensor. I disconnected the hose at the pressure sensor, gave a big blow into it by mouth to dislodge the foreign material, (blow until you hear bubbles in the water) and it has worked fine ever since. After clearing the tube, you need to pump out any water in the drum and then reconnect the hose to the pressure sensor to get the correct empty pressure.
Big al, October 2008