GA-P35-DS3L: Upgraded Q8400 Freezes Constantly

[Fliko]

I've updated from my ancient C2D E6400 to a Quad Core Q8400 processor, but my computer constantly freezes within a few seconds of startup.
I've updated my bios to F9, and my computer works fine if I stick the E6400 processor back into the computer.
I've also checked the power supply and everything seemed fine, except the ATX connector +12v rail was putting out 7V (I checked the voltage while it was on, and frozen), however all the other power readings (including the +12V) are better then great.

Motherboard: GA-P35-DS3L
Bios Version: F9
Processor: Q8400 (Use to be E6400)
Power Supply: OCZ ModXStream Pro 600W
Memory: Kingston KVR533D2N4/1G (3 Sticks)
Video Card: ATI HD3850
Soundcard: AzureTech XRaider 7.1
Running 2 Hard Drives, 1 DVD Drive

I've loaded optimized defaults, and have also tried disabling C1E, forcing CPU Vcore to 1.3250V, unplugging all USB cables.

Computer seems to freeze if there is keyboard input, or it freezes either at Gigabyte POST image screen, at "PCI Devices Listing ..." screen. I've managed to get it to "Windows 7 Starting" screen about twice by random chance.

EDIT: I've cleared the CMOS many a times.

[Fatman]

Try reseatting your cpu? Is there enough thermal paste on the cpu? Not a heat problem?

[Fliko]

Yes I've tried loading optimized, just tried it again, and it still doesn't work.

Computer seems a touch happier though, it will consistently get to "Starting Windows" screen, and then freeze (Doesn't seem to even access the hard-drive after that). If I try to enter bios by hitting DELETE on POST image screen, then my keyboard seems to reset (all lights go off then on), and then I either get a black screen with the computer still running, or a black screen with a little flashing line.

EDIT:  Nevermind it still freezes randomly within a few seconds of startup, or during a keypress
Both of these situations do not happen if I put the older processor back in.

EDIT #2: Appears to be enough thermal paste, and I've reseated the processor several times during switching, I'm sure it's seated right, I will try removing and reapplying thermal paste to the CPU though, but it looks like there is the right amount on the chip.

[Fliko]

Wiped off thermal paste and any oils, and reapplied a rice grain sized dot to the CPU, and let the heatsink spread it and I still have the same problems.
The motherboard seems to read the processor during POST

[Dark Mantis]

I would say it is probably a PSU problem and the older CPU works ok as it doesn't draw so much power from the PSU but as soon as you install the new one the PSU gives up.

What make/model and age is the PSU?

[Fliko]

I would say it is probably a PSU problem and the older CPU works ok as it doesn't draw so much power from the PSU but as soon as you install the new one the PSU gives up.

What make/model and age is the PSU?

OCZ ModXStream Pro 600W, it's about a year old. I checked the voltages it was putting out with a multimeter and they seemed to be in the nominal range, and the readings from the motherboard in the bios seems to agree with this too. The only oddity was reading 7V on a single +12V rail (Computer was possibly trying to save power? All other 12V rails were perfectly fine), which seems to be the only thing I can find wrong.

I am going to look into purchasing a PSU to see if I can get it working on a new one. Is it safe to run PSUs without being mounted temporarily?

[Dark Mantis]

It is safe to run a PSU without it being mounted certainly but you must have a load on it of some description even if it is only a fan or two.

The power supply should be large enough and in reasonable order being only a year old but it could be faulty.

See if you can just borrow a PSU for testing the system first.

[Fliko]

Stuck another PSU in the machine and still no luck. I don't think I would be running over the wattage of the PSU anyways, I should be hitting about 450W whereas my PSU supplies 600W and seems to be fine as far as I can tell from multimeter readings.

[Dark Mantis]

Ok, but 7V on the 12V line is not ok. It is a problem.

Try disabling all the power saving features in theBIOS and see if that helps. C1E, C3 etc