Strange things happens.

I'm starting my site and it writes Site off-line and:
The mysql error was: Access denied for user 'correctusername'@'localhost' (using password: YES).

Then most interesting thing happens: when i go to see what is wrong with the DB with myPHPadmin, everything magically becomes OK, website is working! Then i'm working with my site, then i visit it next day - Site offline again!

Maybe this is some trouble with settings.php, ive set rights to 664.