This just happened to me a couple of months ago. I think when things started getting cold and parts start sticking. If you look, you will find threads addressing this. The one I found worked. It's probably the sensor in the driver's door, since that one gets used the most. It gets stuck/corroded. Also go to V8SHO.COM and search. You will find photos of the parts of the door latch that need lubricating.
The answer is to soak the area with electrical contact cleaner and/or WD-40. I used both. I removed the door panel and cut out the plastic waterproofing so I could get a good shot. I then emptied a small spray bottle of electrical contact cleaner from Radio Shack and WD-40 until it started dripping out the bottom of the door.
The fix was not instantaneous, as the stuff has to soak in. It started improving the next day. I would say the door switch was back to normal within 72 hours.