I'm not going to tell you to go out and buy a new psu because you still may have problems, I was just wondering if you had another one to test with. I just got the Doom game two days ago, performance is great and has beautiful graphics, I'm not having any problems with it.
It seems to be optimized very well for pc and your Mars 760 should run it fairly well. The Mars 760 is a 4GB gpu but that's 2GB for each gpu, once the 2GB of Vram is used up performance will suffer. The recommended gpu is a gtx 970 and the Mars 760 should be close to it performance wise.
I found a review of the Mars 760 and here it says the minimum is a 600w psu. At the bottom of the Hardware Setup - Power Consumption page it says what would happen if your psu can't cope with the load and crashing is one of the symptoms.
http://www.guru3d.com/articles_pages/asus_geforce_gtx_760_mars_review,7.html