I should have listed my clock settings, which I pulled from the mining hardware page. I have both the 5970 and the 4830 set to 850/300 for core/memory. In addition to trying to pull the 4830, I'll try mining at stock clocks to see if I can reproduce.
When dealing with instabilities turn everything back to stock. Mine with single card for 4+ hours. If it doesn't lock up add a card, then the third card. Given plenty of time to test between changes. If it works w/ all 3 @ stock start overclocking.