Machine suddenly shutoff and now won't power on. Followed steps in tech manual; red LEDs flash when power connected and STBY yellow LED lights when diagnostic button pushed. Nevertheless, I replaced the power supply and it did power up and boot normally and stayed on for about 8 hours before I did a manual normal shutdown. Restarted the next morning normally then another abrupt unattended shutdown while idle after about an hour. Now won't start at all. I have taken it down to almost (everything except removing the CPU from its socket) a minimal configuration per the manual and still can't start it even from the jumper pads on the back plane. So it must be the backplane or the CPU tray, right? I am guessing the backplane board as I remember Apple replacing that during the 4th year of ownership. Is it worth replacing the backplane in this thing or just getting a used 2010 for about $500? Will the 4,1 2 x 16gb 1066mhz dimms work in a 2010? It is a pity to trash this machine as the chassis alone is such a beautiful and well made thing.