CD-R's suddenly "unreadable".

You should probably clean the laser. The pits are not as deep on CDR's and therefore are harder to read. My Sony is not supposed to play CDRW's but it does but it occasionally has to be cleaned in order for it to read them.

This is an elusive problem because it can be a lowered output from an aging laser, misalignment, tray movement problems or the fact you are burning the CDRs at 16Xs or 24Xs which seems to present problems with some players. I would first have a tech go through the player and establish output, alignment and all before blaming the CDRs.
Good Luck.

It's very easy to clean the laser with a cleaning disc or remove the cover and clean it with a QTip. Then try the other fixes if it doesn't help.

I burn a lot of cd-rs for the car. my head unit (blaupunkt) heats up during use and often makes my cd-rs unreadable halfway through. I believe instability of cd-rs are to blame. if you examine them, they are thinner than normal cds. the maxells i've used lately seem to have a half-life of only a few weeks of car use, the philips cd-rs seem to fare better. i have to burn new discs every month or so...btw, i also have a cec tl-1x and it plays 'new' cd-rs just fine--as long as they're less than a few weeks old.

Seems like Maxell does cause more problems than most CDR's.
The Imation discs seem to work well and are cheap at about .30 each. I also like the Fugi's.