The other smaller tubes in both the power amp and preamp have more hours on them but according to ARC, need only to be changed after two cycle changes of the output tubes. That is, about 4,000 hours.
Keep in mind that any electronic or mechanical component, be it a tube, a capacitor, a CD transport mechanism, or anything else, can develop a problem (intermittent or otherwise) at any time. While admittedly that stands a greater chance of occurring at times that are either early or late in the life of the part or component, compared to in between, the multitude of possible explanations that have been suggested should be weighed against the likelihood that the sonic effects of an interconnect cable would fluctuate to the extent you have described, after 500 hours. I'll add, btw, that I've had brand new small signal tubes abruptly fail after as little as 2 hours.
Given that you used the CD player for burn-in, yet another possibility, in addition to those suggested in the posts above, would be dirt accumulation affecting its optical mechanisms, and/or wear and tear on its optical or mechanical mechanisms. Either of those possibilities could conceivably affect electrical noise conditions in the player, jitter, or the need to utilize interpolation to correct read errors. Perhaps intermittently.
Interesting input from Stringreen. As someone having significant experience with radar equipment, and although most radars transmit at frequencies well into the gigahertz region, I wouldn't rule out the possibility.
In any event, good luck with the investigation.
Regards,
-- Al
