Sounds like you've got a faulty element in your circuitry. Unfortunately, it's not only hard to identify, it's also intermittent. Could be anything from a malfunctioning integrated circuit semiconductor to a bad solder joint. It's not even clear that the fault is in your CD player. (When you get the shelved down highs playing CDs, does it also sound that way when you switch to other sources?) Reminds me of when one of my computer monitors would occasionally tint the picture yellow. I found hitting the side of the monitor with the palm of my hand always fixed the picture. No way to find the problem source, nor any need to, so long as my palm was available. Along this line, think about some way to spark your CD player whenever you lose your highs. Good luck.
What the Hell Happened?
Last night I was listening to my system. As happens occasionally, the top end was shelved down from normal. Some high freq. air was missing, music was a little closed in sounding. Like I said, this has happened every few days for about 2-3 months. I've always thought it was either unidentifiable audio gremlins or my hearing was a little congested that night(I am 51 yrs old now). However, when I picked up the CD case for Someday My Prince Will Come, Miles Davis, my finger touched the metal top of the CD transport and there was a slight "snap" sound and immediately the music opened up! I could easily hear it while standing in front of my equipment stand! I sat down and listened to the last two CDs I'd played and all the high end extension and air was back, and it stayed the rest of the night!! Anyone have any idea what happened and how I can prevent it or correct it when happening again?