I had a similar problem when my dishwasher was about two years old. I found that the piston valve was corroded and did not seal when activated, allowing water to come back into the dishwasher when pumped out. To check this valve, remove the filter screen in the very back of the inside of the dishwasher. The piston valve is right below the screen and can be unscrewed with your hand. If it appears defective - replace it. This fixed my problem.
Tom
January 2006