Originally Posted by Eggplant Jeff
The locknuts are tight, the wheel didn't move that I can see (no fresh scrapes in the paint).
Its still probably the wheel moving slightly in the drop outs instead of the cog slipping. Going with what you said about it slipping when you mash hard, the feeling you skipped teeth, then the chain being slightly slack all points to the nuts slipping. They dont need to move much to make the chain slack and the dropout would probably still looking like only one impression was made. If it was the cog it should slip when you apply back pressure then again when you mash and keep doing this, it also shouldnt make your chain slack. if you stretch a chain enough to make it slack in a 4 mile ride then you should throw it out.