WordPress 3.1 database unstable – category disappears

a view days ago, i just find something weird in my blog. after upgrading from wordpress version 3.0.5 to 3.1, all Category disappear for no apparent reason. more surprising thing, sometimes it appears to Category suddenly and disappear again, and so on. and I concluded that WordPress 3.1 database structure is unstable.
until this moment I am still seeking how to restore to its original state. because after the upgrade to version 3.1.1 did not solve the problem.