you are not the first to have similar-ish problems. in the past, it has always come down to something with the hardware... power is common issue. bad risers another. another is just crap cards or motherboard that can't handle the load/timing/something for whatever reason.
best approach is to remove all but one card. seem how it works, how far it can go to stable. then add one, and so one. find out what makes things go down hill.
Could be that my mother is not so good to handle x 5 card overclocked ? because individually the card work fine, together not... and i each riser and its OK.
It's hard to say.. there are two of you having trouble with 5 cards, not sure if both on the exact same MB.
What we know and questions..
Things work fine with 1 card (btw how many cards does it take before things break?)
It's probably something do with the motherboard, I think we've done enough to rule out power or thermal issues
There are some motherboards that working fine with 5 gpus, but don't know what model(s) that is either
Could it even be related to the specific GPU? Like would 5 5830s work, but 5 6950s not? Anyone got a big enough farm and one of these boards with issues that can test stuff like that?
Maybe it would help if people posted the motherboard and GPUs that they have tried and what the results were.. I'll start
MSI 870-G45 - 3 GPUs, works fine with 5830s, 6870s or 6950s
MSI 870A-G54 - 3 GPUs, works fine with 5830s, 6870s or 6950s
Dell SC420 mb (don't laugh) - 2 GPUs, works fine with 5830s, 6870s or 6950s