A couple things:

You shouldn't need a dummy plug for 290x so don't bother with that.

The SDK software is of no use to you if you're using Catalyst 13.11 Beta or 13.12.

If you're eliminating all peripheral components(including other video cards) and risers, doing a power drain, wiping your drivers with either Display Driver Uninstaller or a sweeper and reinstalling and still having problems, you might have something corrupted in your OS or a MoBo issue.

Both of those issues would typically give BSODs.

If you're not getting any crashes, just weird hash rates, try a different cgminer version. Don't forget to eliminate cgwatcher for now and don't use GUIminer ever.