A common problem with 'generic' PSUs is that even the high wattage ones do not put out consistent voltages/amps when under increased load. They work fine under normal load. I would say it's definitely your PSU.
I would suggest purchasing a known brand like OCZ, Corsair, or CoolerMaster, that have a 80+ Bronze rating (higher, if you're using extremely expensive components). Any of their 600 W PSUs will provide better performance than a generic 800+ W PSU.
You're also running the risk of damaging your components.