you probably need to make sure that your cog is on tight. i stripped my hub on my khs and had to buy a new one. as onetinsloth suggested, first put the cog on with the lockring and go ride ride for a bit with no backpedaling. second, come home and then retighten the lockring. maybe your cog is low quality? the cog and lockring are two things i would never skimp on since they are my lifelines.