Not BSOD, but random crashes (Browsers(flash) the culprit?)

Hello everyone

im having an issue which i cant solve for quite some time,

my laptop will crash every now and then and it usually happened more than 3 times per day.
it is not an overheating issue as i had replace the thermal paste and clean the fans as well as the air duct, both fans are working properly.

i have tested my RAM using memtest86, however no errors were shown after 5-6 hours which i managed to pass 5-6 passes.

i noticed my laptop will crash whenever i watched a video (VLC with MKV or MP4 format) or i opened browsers(firefox and chrome) for quite some time. i noticed when my laptop crashed, usually i have a tab or currently playing youtube videos
my laptop is RARELY crash when i played games such as DOTA2 or Diablo3, it happened every now and then, but the ratio of my laptop crashing while playing games compared to watching video or steeaming youtube can be 1:10

there is no dump file created as it is not BSOD, my laptop just crashed and didnt restart, i need to boot it up manually as if someone just unplugged my cable from the wall.

i'm willing to do any test suggested, currently im doing Prime95 CPU Stress Test, but still no errors after 2 hours, im planning to keep it stressed until 6 hours or more.

please help me in this matter, thanks!

*bump*
hello anyone??

Tried updating gfx drivers at all?

yeah, just now i updated my graphic card to the latest one, 331.40 (Beta) Release date 30/9/2013. Still crashing
if you wanna ask me about other drivers, yes i've updated 90% my drivers to current one using Slimdrivers and Drivermax.
some of it i choose not to install such as HID mouse compliant driver which rendered my mice useless, so i stick with the previous driver.

i've done the CPU Prime95 Stress Test for more that 5hours without any Errors message shown. I've noticed if i left my laptop idle without opening any applications, it is rare for a crash to happen, it must be incompatibility issues in software or hardware somewhere. But i dont know what it is.

i've formatted my laptop and the issue still happened, im lost. for your information, sometimes the laptop crash beforei can see the windows Flag, but if i boot it again, i can enter windows without any problems. any ideas?