This is interesting - and not very good news for you. You'll need to seriously think about replacing your PSU.
If the people over @ Guru3D are correct
Jewel Black SP-ATX-420WTB-PFC-1 has only about ~15Amps +12V.
That is really low (and I mean REALLY low) amount of +12V power for a 420W PSU. Something we'd expect to see in a 300W PSU. For a decent 420W PSU we'd expect to see 24Amps or higher for +12V.
Here is a budget $25 300W PSU: FSP Group ATX300-PA 300W
And it has 22Amps of +12V. (12V1+12V2=264watts Max / 264watts divided by 12volts = 22Amps)