Try the old GPU to rule out GPU, then suspect PSU.
The old GPU is 9500 GT and it does not draw much power and does not require 6 pin connector. If the 9500 GT worked properly, then is that enough to rule out PSU problems?
Secondly, keep in mind that the problem ONLY occurs during gaming, it NEVER occurred when doing anything else, can a defective PSU cause such a problem?
As a note I tried to connect the card to the power supply via the native 6 pin cable and via the 2 molex to 6 pin adapter, the problem occurred in both settings.
I am not trying to say that the problem is not the PSU, but that is what I am thinking about.
Thank you for help