Whirlpool Dishwasher won't drain during normal cycle

I have a Whirlpool Dishwasher Model # DU1055XTVS which is about 4 years old.
It runs through its normal cycle and cleans the dishes well. However, at the end of the cycle, it appears that it doesn't empty the water basin. The tub is always full of water at the end of the run cycle.
Every morning I have to push the Cancel button on the dishwasher. When I do this, it goes into a 2 minute cycle - where the first thing it does is empty the water. I can hear it running out through the drain, and it sounds fine. After the two minute cycle, the tub is always empty. Without fail this works - so at first I thought the drain might be clogged but considering this always works, I don't think it could be the case.
Interesting thing I have found. At the end of the dishwasher cycle after the heated drying has stopped, I can immediately push the Cancel button, and nothing happens. I have to wait 1/2 hour or so until the dishwasher cools down, before pushing the cancel button to get the water to drain. It seems that while the dishwasher is still hot, it won't empty the water or process the cancel button push.
What's the problem? What should I look at or replace ?