first of all, don't run them in parallel, you cannot crossfire two different cards.
second your PSU is undersized for your system
and why is it that you have to load the iso of the game through magic disk? hmmm?
1. it doesn't matter...it's a multi-monitor setup, and only 1 card is 3D-active
2. the PSU should be good if it indeed can supply 280W. My i5-2500+5850 never exceed 300W (I'm talking about Skyrim, DeusEx HR). But the branding could be problematic. Anyway, from my experience, PSU problems usually generate memory issues and resets, not stuttering.
3. I also use daemon-tools for my ISOs. That's because I did not want to replace my damaged internal DVD drive and I make (note: MAKE) my ISOs on my laptop. As long as he does not mention cracks or downloads, there is no proof of misuse.
I also had stutters from vsync and AA settings. Other things to check are anti-virus, download apps, malware. Also HW supplied CD/DVDs can contain SW which do "bad things".