loose rockers

alright so I'm havin a problem with my rockers, my engine is a later 69 302, i'm running a crower cam with just a lil more perk than a stock, the rest of my valve train is 100% stock, my problem, the rockers on exhaust keep coming loose and then popping off the valve stems after about 5 minutes of running, when this happens the exhaust has no place to go but right out the carb which is annoying and a fire risk.... I replaced the rocker nuts(5/16s) and it still happens, so now I'm gonna replace my rocker arms(which are the original ones that i never had problems with before til i rebuilt the engine), also i noticed that when i go to tighten down my rockers the nut will bottom out on the studs sometimes before the rocker is fully tightened.... anyone have any idea what this problem could be?

thanks, i got the solution though, i went to a classic mustang restoration shop here in san diego, and the head mechanic there said on a 69 the rockers aren't supposed to be adjusted like the book says, but instead are supposed to be tightened to the stud all the way, which really fixes my problem, car runs like a champ now...