How much amps are on the +12v rail?
Getting a molex to 6pin adapter might work.
Buying a new PSU would work... Thermaltake TR2 Series TR-500P 500W, 575w, 600w, Single +12v Rail or Dual +12v Rail Etc...
If the Video Card isn't getting the recommended Power/Amps via the +12v rail you will get constant rebooting / no booting or constant errors...