I like the keychain myself, guess it tales all types . Two things spring to mind.

1) In the latest version of keychain access on 10.3 there is a keychain repair tool, under the window menu. You can verify and / or repair your keychain if needed. You can also create a new 'factory settings' keychain using the options button.

2) Keychains can be set to lock after periods of inactivity which would cause you to be prompted for the keychain password next time an application, such as mail, tried to use it. Look at edit:change settings for details.