I was wondering why Classic was failing on my machine -- causing a hard lockup -- and perusing the Apple support discussion groups gave me a big hint: I had v1.3 of the Keyspan USB serial adapter for OS X installed, and for reasons not quite known, this driver prevented Classic from starting and/or caused the crashese.
The fix? Go to Keyspan's website and get driver version v1.4b9 (only for Jaguar, not 10.1.5 or earlier!!). A quick install, and now I have Classic back...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020831084503544