STG-58 carbine troubles?

[BATS]

i have a dsa carbine built from a stg-58 kit, i have had it for about two years. it has a problem. every now and then, it will fail to go into battery like the bolt carrier is dragging. it only does it from time to time. it has been sent back to the factory, they say all is well. i have used all different kinds of ammo, mags, ran it dry,well lubed, over lubed, you name it. i am at my wits end and i am ready to sell. does anyone have any ideas. i am stumped.