I have the same problem with Linux, but not every H81 BTC Pro board. ccminer crashed when using 6 cards and only with a few algos, e.g. JPC,NIST5.
X11 works without problem. So I found out that not every H81 BTC Pro Board has the same build quality. The reboot indicates that something wrong with the power connects to the board.
I know it's really stupid but I tried a lot of combination last months to find the problem. (I have 6 H81 BTC Pro boards).
I'm thinking about buying a H81 BTC Pro and hopefully I don't run into any issues.
Did you also give power to BOTH of the molex connectors on the motherboard? I know some people say "you don't need to if you're supplying power to the risers!" but it's always good to plug it in and test in case you haven't.
I run a H81 BTC Pro with 6 EVGA 750Ti's (without the 6-pin PCI-E power), I have 5 cards on 16x-1x powered risers, and the 1 card in the x16 slot on a 16x-16x unpowered riser. Rig has been running flawelessly for nearly 4 months now!
Of course I need to plug two molex connectors to the board otherwise the board autorestarts itself if you run ccminer/cudaminer for every algo.
I have 6 of this board and only two of them still giving me trouble. The worst algo is JPC in my experience.
I use for all card USB power riser from same brand, but I notice from those Chinese quality you can't expect much.
I still have found out the reason for these 2 trouble boards, but from my test I can conclude that ccminer/cudaminer works fine.
thanks guys. sounds like powered risers may be the answer.