I think what you are facing is a very elusive bug. I had the same issue for a while. I thought it might be the nVidea graphics card, but when I got my new computer, it had the same problem with an Intel graphics card.
I went back to 12.04 on my new computer and left 14.04 on my old desktop. I discovered that some time after I have been on the internet, sometimes between minutes and about two hours, the computer would systematically freeze up. First, I may or may not notice the mouse getting sluggish, then, the mouse would not click. I could move it, but the screen was pretty much frozen.
Later, I discovered that I could often break it by pressing ALT-CTRL-DEL and then ENTER. That would log me out and I could go back in.... sometimes.
This ability leads me to think that the graphics is frozen and not the computer. At least until you have moved the mouse and clicked a bit after it freezes.
I also suspected Firefox for a while but discovered the problem occurred with Chrome as well, and also it occurred if I never used the browser. However, when I moved my email, etc. to my new computer, it ceased to crash unless I accessed the browser. Later, I left it running without using the browser and eventually checked email with it. About an hour after setting up email, it froze up again.
I since disconnected it from the internet so it could do some disk rescue. It ran continuously for four days - a record! Once I connected it and did the updates, it crashed within 2 hours.
I am not the expert, but the way I interpret my experience is that there is a bug that is somehow related to connecting to the internet. The more I access the net, the more likely it is to occur. The first effect I sometimes see is the mouse stall a little. Then, the OS no longer updates the screen, just the mouse. Finally, the whole system freezes. It isn't directly related to nVidea unless it is also related to Intel graphics (different drivers.) It isn't related to Firefox, Chrome or Thunderbird. But, I may have not ruled out any association with updates. It definitely seems to be related with connecting to the internet.
I recently re-purposed my 14.04 drive so I no longer have 14 installed.
I changed back to 12.04 where I no longer have the problem.
If you search for 14.04 freeze, I think you will find a lot of others having similar problems, but no solution.
I hope that the information I just gave can be used by someone who knows a lot more about programming/debugging Ubuntu to help find the solution.