If you have a CDR210, it's probably dying on you. Mine did this all the time, and wouldn't really play any kind of CD unless the interior was nice and hot (which means it didn't work at all, even in the mild winters we have here in Houston)
You should consider buying a CDR220 (NOT a CDR23 as it won't work in your car) or an aftermarket head unit. The 220 will maintain the look of your dash, but is still a Becker and not that great of a unit from what I've discovered. Here's one I just found on Ebay for you:
http://cgi.ebay.com/ebaymotors/Porsche-CDR-220-Stereo-CD-Player-Boxster-911-986-NR_W0QQitemZ7992935707QQcategoryZ38641QQssPageName ZWDVWQQrdZ1QQcmdZViewItem
I have an aftermarket Alpine wired into my factory 4 channel amp and it's amazing... pumps out far more sound than the 210 due to better electronics (not more power) and plays MP3's burned onto CDR's... which means I can squeeze 14 albums on one CD at 192k, the highest MP3 level of recording.
Just a thought. Do try a CD cleaner as Richard suggested, but don't get your hopes up. The unit is very old and has been exposed to sun, road grime and fingers... lots of top down driving is murder on car stereos, dashes, leather interior pieces, etc.