Clean all the cosmo out well, especially in the gas tube areas. Both of them. The long tube over your barrel and the one under the rear sight. Hot water, brake clean, parts cleaner, what ever it takes.
Another area you really need to clean is the bolt and firing pin. Remove the bolt and remove the pin, make darn sure they are spotless. When you install the pin back into the bolt, shake it back and forth with your hand. make sure the pin rattles freely. If you do not do this, the pin can get stuck forward and when the bolt cycles, the stuck pin will hit the primer enough to send the next round off.
This condition is known as a "slam fire". The weapon will continue to fire at full auto rate until the mag runs dry. You can do nothing to stop it but hang on till it quits.
Dont take me wrong, the SKS is a totally safe weapon issued to many many peasants around the world. Once clean of the cosmoline, they only thing that can stick the fireing pin is a piece of broken primer,usually from useing Wolf ammo.
Not all aftermarket is junk, I perfer the Tapco 20 round mags to the steel box.
Ive out fitted a few with different plastic stocks to make it fit me better, the origional stock was made to fit folks that are only 5 feet tall.
The trigger is horrible, this can be improved.
Ive competed in many Military rifle comps useing SKS rifles against Ar's,FN-FAl's,SIG's ect ect.
Its a more accurate weapon then folks give it credit for.