Drop the two or four bolts on your steering column, roughly where your knees sit when you are driving, On the top side of the column is the actual switch. They do not fail very often, but they DO fail. My guess is that the switch is sticking half way between positions.
If you want to run a cheap test of this before you buy a brand new switch, then go to the bone-yard and pull one to test.
If this isn't it, have the starter tested.