I am 99.9% sure it's the cog and lockring. Tighten the cog as tight as you can, use any mechanical advantage that you can think of(breaker bar), you won't over tighten the cog. When I started out I used to place the front tire against a wall and stand on the pedal quite hard to make sure the cog was tightened properly. Once again, it is nearly impossible with common hand tools, no hydraulics allowed, to overtighten a cog. Then tighten your lockring, make it pretty f'ing tight. Not like putting the top of the peanut butter jar back on tight, but " My Park wrench has broken the skin" tight and you won't have any more problems. Before you do any of this check to make sure you haven't stripped all the threads form the hub.