Physx setup failure (sorry if this is posted twice, it didn't show up the first time) Need help

So everyone knows, I have an Sapphire motherboard with an ATI chipset, which allows the 2nd version of Crossfire (so it has 2 pcie 16x slots). Based on my knowledge, Physx is not reliant on manufacturer company, the slots used, or whether the cards are bridged.
I have a geforce 8600 for my main video source and is in the master pcie slot. Since I have a spare pcie slot with a current nvidia card, i figured i could just get a 8400 for physx. Like I said, I’m 100% sure the card used for Physx does not need to be bridged, and it is not. Anyways, I’ll state all of the known facts of my current situation below:

*If I stick the card in and enable the slave slot, my computer repeatedly restarts within 1 second increments and never gets anywhere.
*If I take the card out, it works again. When the slot is disabled and the card is in it, the fan spins and the computer starts up but the 8400 is inaccessible.
*The 8400 DOES work, I tested it by itself
*The slave pcie slot also works, because if I put either video card in it by itself, I get an image.
*I set both slots to 8x, since thats the recommended multiplier for my mobo when using 2 cards. (even when trying 1x, it still doesn’t work when enabling both slots)
*My PSU is 500W and is around 4+ years old. According to Speedfan, one of the 5v rails is well below 5v - its even gone to 1.8v.
*As far as I know, the 5v rail is used for the pcie slots, so theres a possibility my PSU is dying out, but I do currently have an overclocked and stable system.

As of right now I don’t have a replacement power supply, so I was wondering if anyone here had any idea of what else could be the issue, or if you think the PSU is definitely the problem.