Beef Jerky too Tough

Help! I've been making jerky for a year now and have flavor just what i want but jerky is so tough some times it nearly impossible to chew. I usually use eye of round or top round cut across grain 1/4" thick and smoke in YS 480 till bends and cracks but doesnt snap at around 165 degrees. Jerky usually is so fibrous that you can't bite off a piece and gives your jaws a real workout. I know jerky is suppose to be tuff but sometimes mine is rediculously tough. I was wondering if maybe i'm over smoking or under smoking? I buy jerky from a place in Utah that is thicker but just almost falls apart like a over cooked brisket when you bite into a piece, i would like to duplicate. I'm at the end of my rope would appreciate any help or comments from someone.

Thanks,
Stan