Originally Posted by Dr.Grace:
I've had this same problem ever since Palm "updated" me a second time to 1.21, even though I was already on 1.21. Now, every time I leave Classic open, either in the foreground or background, when the Pre goes to sleep for a while, then I wake it up, Classic is frozen.
Update to Classic 2.01 did NOT solve this problem for me. 
Updating to WebOS 1.3.1 fixed it for me. Classic is now pretty stable, at least for the apps I use.