DELL XPS 8700 monitor flickering and pc freezes

I bought a new DELL pc from local dealer in May 2014
 
DELL XPS 8700
with AMD Radeon R9 270 -
i7-4770 CPU @3.40 Ghz with 16 Gb RAM -
Windows 8.1 64-bit operating system

in the past two weeks the monitor starts flickering and the pc freezes. Only by depressing the power button I can shutdown and then restart and after a couple of minutes the same thing happens.

I have run diagnostics and there is no problem,
I have checked device manager and all hardware seems working okay.

When reading email or reading this forum nothing happens.
But when I watch videos via facebook or youtube or play Dreamworks Dragon: rise of Berk on Facebook with my kids, then the problems start after a half hour.

I have uninstalled, deleted and reinstalled newest drivers for the AMD Radeon card, but that does not fix it.
Also when I open the box, all three fans the Dell came with, are working fine and there is no heat coming off the video card. The only heat is generated from the hard drive compartment.

Please advice what to do next?

Thanks in advance.