Does the timer actually advance to the off position or does it stop short of the off position?

There is a possibility your start button may be stuck or fused in the start position.
How do you get it to stop?
By opening the door or disconnecting the power.
Does it start back up by itself, when you either shut the door or turn the power back on?
If its off when you turn the dial does it start without having to push the start button?

Ok Do you happen to have the wiring schematic for your machine? It's usually located inside the control head. I am leaning toward the timer however I want to look at your specific wiring layout to see if something else might be causing the timer motor to continue. I think though with what you said we can rule out the start button.
If you have that schematic take a pic with your phone and attach it here so we can take a look. Just use the Go Advanced button below and then click manage attachments. You will see how to upload the pic.....