Wouldnt boot (white screen), Now wont accept password

I previously had the white screen with a caveat, it would cycle / flash the white screen, then my desktop, and back to the white screen, yet would psudo-boot and only display the white screen. Psudo-boot means when it would flash back and forth between the white/desktop, I would get a notification on another computer from TeamViewer that my MAC was online....

I was unable to log into my Mac via the other computer thru TeamViewer...

Ive cleared the PRAM, run ALL the options on the Disc Utility (verify/repair etc), reinstalled the OS.

Now, I get to my log on, type in my password, it appears to accept it, yet returns to the log on.

When I enter a password I know is incorrect, I get the 'wiggle' to tell me its wrong.

Now - During the DIsc Utility 'verify permissions' - I get the following message;

" SUID File "System/Library/CoreS...." has been modified and will not be repaird

~So, I am at the log on, but the compuetr seems to reject my password.

Any suggestions before I take a hammer to this?

thanks in advance