I used to have these odd total-freezes. Entire system would lock up. I *thought* it was a faulty on-board NIC, but that was only the symptom. Once I got a good PSU, the NIC worked fine with no problems. It's not that I needed more total W, just that the unit I had was a total POS, through and through.
What brand PSU do you have? Also, can you check your voltage meters, if you have any? I really think it's a power issue now, but this is harder to test for and harder to prove.
Keep in mind the old 5000 series cards were really stripped down and barely used any juice at all.