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.
Thanks for the tips, I shall give those things a try and see how I get on.
Update: I seem to be getting on ok now, not great but 800 k/h. I started with 1030mhz and intensity 13 getting me 500 k/h, I then lowered my gpu clock to 950 and upped the intensity to 20 getting me 800 k/h. Not sure what I can do next but I guess there's going to be a lot of messing around to get the kind of speeds that others are seeing with their 290Xs.
Thanks to everyone for their input, probably would have given up a while back otherwise.
Also, I may be able to help you with tweaking if you like. What brand/model of 290x do you have?