When does the keychain unlock itself?
There is a setting that can lock the Keychain after a period of inactivity.  So if you have unlocked it after sleep and then find after a few minutes that it is locked, it probably means that this option is turned on.



It's also possible that on some items you haven't checked 'always remember' and so it is popping up the request for credentials via keychain.

You can also repair your keychain using this guide.