Is there pitting in the gas tube. I had a yugo with the same problem. A "new" gas tube solved it right away.
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You can check the gas piston length here along with some other info that may help.
http://www.yooperj.com/SKS-8.htm
I was thinking that someone may have replaced the piston with a paratrooper piston. The regular one is 9 1/2" long, the paratrooper is 8 3/4" long.