Buy another 5870, RMA the problem child one. Once it comes back, Crossfire! BAM, upgrade and solution in one.
Also, to actually steer you towards diagnosing the problem, run Memtest86 or P95 (test with alot of RAM usage) or IBT (max RAM usage) and see if that runs okay. Anyone of these tests should hopefully rule out a memory module being the issue, if by memory module you meant system memory and non memory on the Video card. Then, I would go download MSI afterburner and manually set default clock speeds and make sure the voltage is correct. If with default voltages\\clocks set, SC1 and Aero is still giving you issues then you can conclude its not the powersaving declocking of the card. It could be the declocking isn't reclocking back to default clocks which would be a driver issue, then you could either stick with MSI afterburner on startup as a fix or try a different driver version. Hope this info helps.