... its a 500w peak PSU.
Do not use the PEAK VALUE, ever.
The peak rating is meant to account for transient PSU loads, like when the PC boots up and needs to initialize all the components. From the end-user perspective, it is useless.
That's a 450W PSU you got there, and not a very good one.
^Wlow-cost and undersized components (main capacitors rated at just 85°C, bleh) this model suffers from excessive ripple on high loads.
I strongly suggest you upgrade your PSU to something decent and sell the old one on ebay to recover costs.