The odd misbehaving extension has been known to cause memory problems, or take all cycles from the CPU. Windows updates sometimes cause problems.
It could also be a web page that you keep in a tab, that does periodic activity, and has a memory problem. It could be a plugin like Flash or Quicktime, etc.
What have you done different, starting about the time your problem started? You may have to see if you can isolate the problem to what web sites or activities you have done during a session.