Dell Studio 1555 :: Event 6008 - Computer Shutdown At X Time Unexpectedly

I have a 4 year old Dell which I have had cleaned and maintained by Dell services in order to keep temperatures and hardware in check.

However, I am constantly experiencing an "Event" according to Event viewer, or "Event 6008". What it says is "Computer shutdown at X time unexpectedly."

I have monitored temperatures, have all latest drivers for all components, yet I continually get the same crash. The computer goes completely off, no bluescreen, no error. I used "Who Crashed" in an attempt to record a dump file, but it keeps coming empty.

The error/crash occurs only during games, some more frequent than others. I have tried all possible options for those games, low to high settings/resolutions/etc, no background programs running - no difference. 

Some days, the error/crash occurs more rarely when unplugged and running on battery(which don't last more than an hour), and once I plug it in to charge it, it starts crashing in a few minutes.

It is also running Windows 7 64-bit, upgraded from the stock Vista(where the error/crash also kept occuring).

The laptop is a Dell Studio 1555(the widescreen version). I took it to one of Dell's shop for them to "fix it", tho after running all their stress tests they told me there is nothing wrong with the computer. So 7 completely different games crash the computer, but Dell's stress tests fail to identify what is wrong with it?