I was speaking of a new cam. An old cam with new lifters, in some cases, can cause this as well. New lifters material, can be harder than a worn cam. I would venture to say, the cam had a problem from the get go. I never mis-matched cam and lifters. Just me. Always went with new push rods, hardened valve keepers, and roller rockers. Always replaced the nylon geared timing gear, went roller chain... If you have her apart, why not do it right? But, hey, you do what you have to do with what you have. I understand that. It just doesn't always work. My buddy, who was my boss when I was 19, had a screming 302 Mustang. Pull the wheels Mustang. I have seen the insides of that engine many times over my younger years. Real competitor. I was trying to make a big block Camaro keep up...lol...