"I have LP’s bought 50 years ago that play perfectly."Have you ever changed the stylus?
"I have many CD’s out of 400 or so that just wont play - not just on fussy hi-fi but in the standard car CD player"That is what happened to me when CD player's laser started dying.
Are the CDs that did not last original/bought ones or you burned them yourself? Those burned do get messed up after a while. I do not have 50 year old CDs, but some are approaching 30 and last I checked they were fine.