boot into Recovery mode
repair the drive
you could have bad file, preference, trouble with your system
Safari and Safari.web.content (Activity Monitor) can eat up a couple GBs of RAM which should not be a problem assuming you have "enough"
have you tried boot Safe Mode?
Mac OS X & Mountain Lion Community
General purpose Mac troubleshooting guide:
Creating a temporary user to isolate user-specific problems:
Identifying resource hogs and other tips:
Starting the computer in "safe mode":
To identify potential hardware problems:
General Mac maintenance: