If the problem is actually the system bus, yes, you would need to replace the logic board.
I'd actually try out a utility called DiskWarrior and see what it reports on your hard drive. It's possible that your system is just corrupted.
Have you tried booting from another instance of the Mac operating system? I like to keep an external boot drive around for troubleshooting instances like that. Boot off of the external drive, run DiskWarrior and let it try to repair your OS. If that doesn't work, you could try re-installing the OS over top of your data (so you don't lose anything). Then see if it will boot consistently for you.
Don't do anything without a backup of your data. Seriously. Let me know if any of that helps.