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.