Kenmore Ultra Wash III Dishwasher will not drain...

Question

My Kenmore Ultra WAsh III Dishwasher does a good job on cleaning the dishes, but when the draining starts, the water comes out of my overflow valve on the kitchen sink. The counter gets flooded as does the cabinet below. Can't understand why this is happening. The garbage disposal drain is not clogged. Can anyone let me know what would cause this problem?

Thanks,
Curt