Hi,
I've had this problem happen to me twice, and it took me a while to figure out what was causing it, so I just wanted to let others know if they have a similar problem, the fix is not that difficult.
The problem was after upgrading my OS X to a newer version, upon boot up, the computer would boot upto the point where the menu bar would bounce (I'm not talking about the Dock). It would drop down and then jump back up, over and over.
The problem was with Ownership & Permissions. I just happened to have a second hard drive with an operating system, so I was able to fix it by selecting the drive that was giving me problem and fixing the Ownership & Permissions via the "Get Info".
This was so anoying, since the first time I ended up reformatting the hard drive and replacing the OS and all programs. The second time is when I finally figured out what was causing the problem. What a pain the Permissions thing is. It's the only thing I hate about OS X.