User Password Not Working?

My user password in not functioning. If I try to update software or any other activity that requires entering my user password, it tells me it's invalid. However, if I restart my computer it accepts it and I can, for a few hours or days, update software and it operates fine. But then it will again become "invalid" until I restart my computer. I've been working with Apple Tec Support for days, but they are still baffled and are defaulting to a re-install of my operating system, which I'd love to avoid. Anyone have the problem? ...or better yet a solution?

I'm using 10.6.8 on a MacBook Pro.