Bosch Dishwasher fills then partially empties & so on?

My dishwasher fills until the microswitch is activated. Instead of stopping the water intake and continuing it partially empties and then repeats the process. The water matrix; pressure chamber; float; and microswitch all appear to be working normally. All waterways are clear and water enters the machine correctly. I replaced the flow rate meter (from a suggestion) but to no effect. The control module shows no sign of dry joints but it could still be at fault; I cannot test it electrically. I tried the diagnostic mode but it will not proceed due to the filling problem.