I'll try playing around with the 5830 overclocking/underclocking. Usually the second I go under 400mhz memory speed it will lock right up. Maybe try 11.7 drivers as well.
When lowering clock, try to skip past the 400 zone as much as you can, I remember an old setup being weird when I let the slider go around 400, but swing it right to 300 something, and its fine..