The mystery of the 'slithering stones' of Death Valley appears to have been solved-- they got it on video, even:
National Geographic has the full story.
According to [Richard] Norris, a geological oceanographer and paleontologist, Death Valley’s rocks move under a delicate mix of water, ice, sun, and wind. Norris and his cousin were able to document the rock movements by inserting GPS tags into chunks of limestone and syncing their movements with readings taken from a custom-built weather station.
Unlike one of the previous explanations, which had thick ice capturing the rocks and carrying them along like a miniature glacier, Norris said their evidence shows that thin ice floes break up and pile against the rocks. This creates enough friction to cause the rocks to skim across the muddy surface of a temporary pond. If you were there to see it, the rocks would look like ice-breaker ships plowing through sea ice—though in this instance, it’s the ice that’s moving the ships.