Well, i just received the reply from Giga byte's technical support:
Dear Sir,
Thank you for your kindly mail and inquiry. About the issue you mentioned, please try the following steps:
(1) Please confirm there is no insufficient power or overheat problem with your system and VGA card.
(2) Please update your VGA card driver and the games to the latest official version.
(3) If possible, please try to test your VGA card and the games in another system, or test your motherboard with different VGA card to see if there's a hardware problem.
If the problem still occurs in another system, a further testing or examination to your VGA card might be required. We suggest you to contact your supplier (where you bought the product) to arrange further examination and find out the root cause.
Regards,
GIGABYTE
And the test in that other PC (withPSU of better name and power) was negative: the VGA did not freeze during hours.
So I think so the weakest chain in my PC the PSU is. I hope am I right...