System immediately powers down after power on

[Pheran]

My trusty homebuilt PC is almost 3 years old now and has developed a power issue. When I power it on, the lights and fans all come on, but it immediately powers back off after 1-2 seconds. I first thought this was a power supply problem, so I swapped out the Corsair 550VX PSU for a brand new Corsair 650TX, but alas, the same problem persists. Any ideas on what to try next?

System specs:

Lian-Li PC-60USB case
Corsair 650TX power supply (new)
Gigabyte GA-M57SLI-S4 nForce 570 motherboard
Athlon 64 X2 5200+ 2.6 GHz
2 x 1G DDR2 800 RAM
EVGA 8800GTS 512 video card
2 x Seagate 7200.11 500G SATA HD
DVD/CD burner
Rosewill USB 3.5" card reader
3.5" Floppy

I also have a new CPU standing by to beef up the system a bit (Athlon II X2 260 3.2 GHz), but I don't want to change anything else until I figure out the power problem.

 

[Rifter]

I would suspect motherboard next or possibly even the power switch on the case itself. Try shorting the pins with a screwdriver to rule the switch out. Also i would try resetting the cmos and re seating the ram and vid card.

 

[SirGCal]

This can also happen if the CPU is overheating. You might see if the heat sink came lose. Simply take it off, check the goo, make sure it's got a good contact, and reinstall it. Also the above suggestions are also very possible.

 

[Pheran]

After a whole bunch of troubleshooting, I suspect that my motherboard is hosed. I first thought that my 8800GTS was causing the problem, but after more testing it appears that the power-down behavior happens if any video card is installed in the PCI-E slot.