The problem I think is your Power Supply (PSU)
You said you have a 420Watt PSU?
Wattage means virtually nothing, you can have a 600Watt PSU with horrible ratings.
What are the ratings on your PSU?
ie, your hold up time, or output +3.3V@20A, +5V@30A, +12V@18A, -5V@18A, -12V@0.5A, +5VSB@0.8A