I was researching what may be going on here, and I feel like there is a permissions issue preventing the keychain reset from being 'saved', if you will. Here is an article about troubleshooting permissions issues called Troubleshooting permissions issues in Mac OS X found here http://support.apple.com/kb/HT2963
For Mac OS X 10.2 or later, open Disk Utility (/Applications/Utilities/). Select your Mac OS X startup volume in the column on the left of the Disk Utility window, then click the First Aid tab. Click the Repair Disk Permissions button. You may see an erroneous message.
All the best,
What exactly is happening to Mail after restarting? "Stops working" doesn't give much information to go on. Do you get any kind of message or popup when this happens?
If you open Console in Utilities>All Messages, Console Messages or system.log, do you see any entries from around the time this is happening?