Steering slips

I drive a 1997 chevy silverado 2wd, with a 4.3 liter, app. 121,000 miles. About two months ago I noticed my steering wheel slipped to the right. It does this everyonce in a blue moon. I'll be driving straight and all of a sudden it jerks to the right. When my trick is sitting idle I can turn the wheels and there is no slack. But when it does slip its usually in between 25-45mph. No past problems with the truck with the exception of belts, battery, alternator having to be replaced. I was told that my truck had a sensor located somewhere in the steering column/shaft that could have gone bad.

Thanks.