Sounds like some of mine, I think my PSU is a wimp (250W ATX powering 2 hard drives and multiple fans and an ASUS CUV4X board with all card slots except the PCI by the Matrox card full) and plan on replacing the PSU when the PC flea market comes by again with a good price on one.
Go through the case and PSU (power supply unit) and clean out all dust and dirt, make sure all your fans are running, and try again, this often helps with the ones I service.