Some recordings were just flawed at times. Others the problem lies with remastering. I have heard it on other genres as well, it can drive you nuts because you have to play something else to make sure it isn't a speaker issue. When you do, you realize it is the recording, not the speaker. I think you are listening to cd also, which older recordings were not mastered for, unless the original tapes are used. What you get is like the bible---a translation of a translation of a translation....