Have you got a power meter? such as a killawatt or similar.
Have you plugged the psus individually through it(but with the whole system running) to check the power each psu is drawing?
Sounds like you have from your post but I just want to be sure.
Like the previous post, I would check that you haven't got both psus plugged in to the same device somewhere.
Running for 30 minutes could suggest heat build up, is the air flow good?

Baz