Make sure all your connections are good: unplug and replug everything in the Mac Pro. Airport card, hard drives, optical drives, PCI cards, processor tray, RAM DIMMs. If your optical drive is causing weird sounds, remove it temporarily and see if that helps.
Make sure you have enough electricity. Your breaker board in the house will show you the amps of each seperate circuit. Amps = watts/volts. My Mac Pro draws up to 12A, and my speakers (M-Audio BX8a) draw about 1A each. Then there's my DigiRack and LCD and CRT and all the other peripherals. Yet my room's circuit is only 15A ... so I have to be very careful. The circuit for the microwave downstairs is 30A... I was thinking about running up an extension cord and cooking with the gas range. I wonder if dirty power or fluxuating power could cause these CPU Power Management panics. Using a UPS for the Mac Pro is a no-brainer; if you're not, then go get one yesterday!
Check also the heat of your RAM. I put a heat spreader on the DIMM that reported in the panic, DIMM 0 (closest to the outside). We'll see if that helps avoid this ECC read error. Not sure why anyone with this level of machine WOULDN'T put heat spreaders, which are less than $10 each. The user manual of the Early 2009 Pro even shows all the RAM being inserted with heat spreaders attached!! Try it! It's not like Apple machines have never had heat issues before, LOL. Do you really trust some cheap Chinese thermometers inside the DIMMs?