Do you mind sharing the core voltage values in the above set-up?
I also have 4 x 5870 in my rigs, but I keep them at 920/1200/1.1250 - every time I push the GPU clock over 940, they become unstable and lock-up (the memory FRQ does not affect much, but it looks like higher the memory clock, the card is bit more stable)
Stock voltage, but I can't tell you the exact value as they are currently mining. I've flashed them with an edited BIOS, which does not allow changing the voltage. I've tried 990 core which was stable, but speeds weren't high enough to justify the extra power consumption or higher temperatures to be expected. For me 960Mhz core is the best setting.
Is 1.1250 the default voltage or you increased it?