Are you sure the valve spring's are o.k. it sound's like a valvetrain problem? Lifter's, pushrod's , rocker arm,s valve spring's. Have you checked all these out? I was going to say it sounded just like a broken or weak valve-spring. But you said you allready checked them.My friend's Cuda did the same thing and we found 3 broken spring's. Supposedly the spring's were new , but the guy we bought the head's off of just shimmed them.Good luck.