[82 Silverado 4X]

I have a 82 pickup with intermittent windsheild wipers that seem to have a mind of their own. When set to the off position they sometimes start and run until I push the knob all the way over to the left. Sometimes they park where they are supposed to and sometimes they stop in mid-swipe. This has been doing this for almost 6 months. It has the dash mounted switch. Anyone ever had theirs do this?