It might be the Power Supply as quite a lot of the HP Power Supplies are marginal at best and when you start adding additional things they play up.
I had one on my work bench that I was told by HP that it had work perfectly for 8 months with an additional Optical Drive in it and then it started performing this way. They had changed the Power Supply several times the M'Board both Optical's and had drawn a blank.
When I plugged in an Antec 500 W PS it worked perfectly so I sent it back to HP with a note that it was the PS and I think that they kept changing them over till they found one that worked. I don't know what they would have done if it wasn't UG as it had a propriety PS in it and it couldn't be replaced with anything but a HP PS of the same type.
Col