K9N Platinum SLI - Powering up the PCIe cards

Question

Hey everyone,
I've got a K9N Platinum SLI board. This board has a power supply input point called PWR2, which according to the documentation is used to provide power to stable the operation of graphics cards. My graphic card is BFG nVidia 9750 GT. This card has the PCI-Express external power plug in the back, and my power supply has a dedicated (red) connector for this purpose, which I have attached properly.
The PWR2 is currently connected also, into one of the black 12V connectors. Otherwise, everything is running fine and smooth, but I am sometimes (once a day or so) experiencing random shutdowns during intense gaming (playing LotRO). Stress testing does not cause a shutdown: I've ran 3D Mark 2007 straight for 8-9 hours, while having a CPU Burner application toast the CPU at the same time. But no problems whatsoever. No other games exhibit this problem either.
Since I am providing dedicated power through the extra connector at the back of the card, should I leave the PWR2 connector unplugged ? Is there a chance that using these two connectors at the same time causes the 12V rail to overload, and the PSU shuts down to protect itself ? When this shut down occurs, the power LED is left blinking and I need to plug out the wall socket and wait for several minutes before the blinking stops and it starts up again.
My computer has an Enermax Liberty 620W power supply, and AMD Athlon X2 4600+ CPU. So there should be enough juice, at least. I've tried the tips and tricks suggested here. Using USB-connected Keyboard and Mouse does not correct the issue. I've also verified that this is not an overheating issue. My CPU is cooled by a Scythe Ninja tower heatsink and two 12 cm fans. It is possible that the northbride is heating up (it burns the finger on touch when under load), but from the Web, I've understood that the stock passive heatsink should be enough, and that the NB chip can take rather extreme temperatures.
* UPDATE *
Plugged the PWR2 connector out from the MB, and update the BIOS to 1.10. It's working as usual, but haven't ran any games yet. Heavy gaming usually reproduces the issue, so I'll write more when I know more.

Answer 1

Updated information on the case.
I've been running 2 days now with the PWR2 unplugged. I have had zero problems with shutdowns since. The only problem I now have is that the USB keyboard and mouse refuse to work in Windows XP after an initial bootup from powered down state. I have to unplug and replug them at the login screen in order to get them work again.
So, I decided to do a final sanity check, and replugged the PWR2, then went gaming. After 2 hours of playing, my computer shutted down for the first time. Plugging in the cable did not solve the USB problem either.
As of this day, I'm leaving it unplugged. It seems like an irrelevant connector, which just causes problems.
** Update **
After following Bosskiller's instructions on another thread about using the "Clear CMOS" button and then reconfiguring BIOS from scratch, the cold boot USB issues were solved. The USB devices now work straight after a cold boot, with no need to replug them.
I recommend that the PWR2 -connector info is added to the list of known problems in the sticky thread. If you have a GPU card which takes its power from a dedicated rail (and your PSU has a dedicated connector for this purpose), then leaving the PWR2 -connector unplugged might resolve your shutdown issues, like it solved mine.
After all, according to the manual, this connector is used to give more voltage to the PCI and PCIe busses, and some PCI/PCIe cards might not like this idea at all. There is a reason why these cards have an additional power plug on them...