System Freeze - Firefox (YouTube), VLC, Lockscreen

[Harish]

System Information:
-Operating System: LinuxMint Olivia x64
-Graphic Card: Nvidia, GeForce, GTX 645
-Driver: Nvidia, proprietary, 319.49 (http://www.nvidia.com/object/linux-display-amd64-319.49-driver.html)
-RAM: 16GB
-Processor: i7-4770

Error Type: System Freezes, System Hangs, System Hang
Description: The system just hangs up and the screen sits still. Mouse does not move around, keyboard does not work and the caps lock button does not turn on the CAPS led above the num-pad

-> observed frequently when running a video and screen locks/intentionally locked
-> observed moderately when playing video on Firefox (known with YouTube)
-> observed rarely when just playing video
-> observed very rarely when just using firefox