Well, that PSU is only on Tier-4 of the list at http://www.tomswiki.com/page/Tiered+PSU+Listings?t=anon
Your particular one may not be of sufficient quality to maintain your rig. Do you have access to another PSU for testing? If you need to replace it, choose one on Tier-3 or better from that list.
Have your stress tests included your RAM? Bad RAM could cause lockups too.