I have a Toshiba 40x CDROM in my CompPC tower. It does the absolute exact 
same things you describe. The patterns I've noticed are that if I keep a CD 
in it at all times, then it's ok. But if I remove the CD, close the unit, 
then open it to put a CD back in, about half the time it won't take. If I 
open the drive, take a CD out, and immediately replace it with another CD, 
then it will happily access that CD. This happens with CDROMs, CDRs, and 
audio CDs.

-Nick