Oh, I see. Been there, done that. Made out alive, though wasted a lot of money in the process. It's just consumer-grade hardware does surprisingly well these days, a simple AMD FX-8350 gives all the power that vast majority of hobbyists need, supports up to 32GB of ECC RAM, and there's enough 990FX boards out there that work great as hypervisors with Xen/Vmware.. There's also a lot of previous gen Xeons on ebay, 6 core / 12 threads 1366 cpus go for like less than $150 and are still very capable. One can build a very powerful server for under $1k and in real-life applications most would never even notice the difference in performance compared to the latest/greatest top xeon.

Well, if you want that latest stuff and can afford it — then sure. For me it's just never worth it, I do like to geek out and play with new toys, but most fun is gone in like a week after the purchase and then I start to feel like shit, realizing that I could get all the performance I needed for a fraction of the price paid.

You might have a problem with power, afaik your EVGAs don't have 6-pin connectors so they draw all the power through the motherboard, and that's a lot of current. The easiest way to check whether not enough power is the issue is to use a powered riser (usb, or powered classic). Edit: oh, sorry, they do have 6-pin connectors, my bad (cool-looking cards by the way, might be a better option than my gigabytes). Not sure then what's the problem. Try to swap risers/cards and see whether there's a bad riser or something. I've had a problem exactly like this (connected the 6th 750ti to a rig that previously had 5 cards installed, and also had crashes, although I don't remember whether those were ccminer crashes or OS crashes) and in my case it turned out that one of the risers went bad. Those cheap "classic" risers have really bad soldering sometimes, I've thrown away like 2 or 3 out of ~30 I've bought on ebay.