I have never heard of etech power supplies. However, the model # MPT-400 leads me to believe that it is a re-badged Macron PSU. Macron make "average" quality power supplies. They are ok in office systems or family computers that are used for playing some simple games, going on the internet, working with word processing and spreadsheet programs and graphics programs such as Corel Draw and Paint Shop Pro.
The first thing that you notice on the PSU is the low amperage on the 12V leg. On a computer, these days, you need a minimum of 16A on the 12V leg. It would be helpful to have a bit more, say 18A or 20A. But, with a mid-range "average" office/home computer you could get away with 16A. I am convinced that the source of your problem is the power supply. I would have the system tested with another - better quality - PSU
Under heavy processor load, your PSU will become unstable because it will constantly be running at peak (100%) whereas it should be running at max (which is at about 80% of maximum efficiency). Further, the more that I look at the other figures, the more certain I am that they are the peak ratings for the PSU. So, assuming that the figures on the label are accurate for the peak rating, you need to multiply them by .80 to get an idea of the real capability of the PSU. The figures come out too low.
Frankly, I would be looking at changing the power supply. Changing the graphic card is not, I think, going to solve your problem. To confirm what I am saying, have your system checked out with another PSU. But, don't use a generic PSU like the ones that come pre-fitted in most cases. Have it checked with a good brand-named 400W/420W PSU such as Thermaltake, Antec, Tagan, Vantec, Enermax etc.
