Originally Posted by
Scrodzilla
Again, it's most likely the cog and lockring. Unless they are badly damaged and/or installed my a mentally ill monkey, cranks don't "slip".
Even then I don't know how that could happen without the bike falling apart. The chainring is held in place by the chain. The cranks are bolted to the chainring, and even loose chainring bolts would give the system less slack than a properly fitted chain. It's the cog/lockring.
Ride up a hill and stop without back pressure. Or stick your front wheel in a corner and stand on a forward pedal. Then tighten the lockring with a lockring tool.