Bosch S9G6B dishwasher trouble starting cycle

[Justint]

I have a reconditioned German built Bosch dishwasher, model S9G6B, approx 8-10 years old that I have owned for about 4 years. Has run like a dream until the past week when it seems to have trouble starting the wash cycle. It is a very simple electronic push button control which just requires the door to be closed and press the start button to run.  Now what usually happens is the dishwasher sounds like it will start like normal but there is no water running and the machine just seems to churn along and count down but not wash.  Sometimes it stalls mid cycle and the continues to hum but not wash.  When the door is opened the dispenser is still closed and nothing has washed.  The last few times I have opened and closed the door a few times, and rocked the machine back and forth a few times and all of a sudden it kicks in like normal and completes a full cycle as normal.  I am wondering if it may be a fault with a switch as everything seems to run perfectly once it gets started.  Thanks in advance for your help!