Did you ever get this worked out?
I thought about it a bit today in the context of my past Dell experiences. It sounds like you've added some hardware (substitute sound, other hard drives, etc.). I think this is your problem.
From my experience, Dell is ultra-efficient at spec'ing a PSU that handles exactly what they've put in a system - there's often no (zero) headroom for more. I once dropped a second hard disk into an OptiPlex and it fried the machines motherboard. Dell covered it under warranty but explained to me that, even though there was a space for the drive and a molex for power, the PSU couldn't handle another drive and the normal failure mode fried the motherboard.
So I would try one of two things:
1. take all your stuff out and test
or
2. take a second PSU and connect it to your two GPUs. Jumper green-to-black to enable it. Now power the GPUs with the external supply and the rest with your Dell supply and test.