The same exact thing happened to me. It's your lockring. After riding my bike for a week or so, the lockring came loose and scared the **** out of me. Buy a tool to fix it, or just take it to a shop. I doubt it's your hub.

The lockring must have come slightly loose, which is why you felt the wheel 'slip'. The slip is actually the cog unscrewing (when skidding, or slowing down) and rescrewing itself when you accelerate.

What hub-cog-lockring are you using?

-Chris