Fan spin/die cycling.
24 hours later. 2 CPU's (i5 670 and 655K), 3 sets of RAM, 2 PSU's, different hard drives, several GPU's, lots o jumper fiddling, in case or on top. Same thing.
Dead P55 Classy. Now I'm wondering if this thing has been the source of all my i5 670 failures over the last 3 months.
Now to find a replacement........someone's selling a Foxconn Katana Inferno for $95 shipped. Should prolly nab that for the kid's rig while I ponder whether I'm brave enough to try to RMA this thing. It's been to to the icy depths of Hades and back - and very much looks like it.