Dishwasher filling with water and flooding when...

Question

My dishwasher washes ok, draining properly etc, but when I'm not using it the bottom of the dishwasher is filling with water to the stage it flows out the front- I think it is coming from my kitchen sinks because when I use them it seems to fill the dishwasher more. I have put caustic soda down the sinks and they seem to drain properly so I'm at a loss as to what to try next.