Has the machine ever worked? It almost sounds like a bad motherboard or power supply. I'd try another power supply if you have one handy. If the machine hasn't ever been working, make sure everything's tight. You never know if the processor had been replaced and is loose or something like that.
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Urban De-Development
* Athlon X2 4200 | 2048M RAM | 160G HD | 7600GT
* C2D T5250 | 2048M RAM | 160G HD | Intel x3100
* G4 1.33GHz | 512M RAM | 40G HD | Radeon 9200
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