You really have a lot of hardware being fed by the PSU.
Disconnect the maximum things you can such as CDRW, DVD one of the HDD (only if you are NOT in RAID) case cooling fans, NIC card and 1 of the RAM sticks.
If the stability problem disappears, the problem would be pointing to the PSU
I assume you did install the 4 in 1 drivers and NVIDIA drivers.
<b>(<font color=yellow>as good as it looks</font color=yellow>)</b>