Not saying your issue necessarily pertains to hardware, but here would be my concerns:
1. Great motherboard for home use. Consumer grade motherboards are not designed for 24/7 365 reliability. As Michael and others have mentioned we have all tried the DIY server route because anyone can build their own PC. The extra features that will never be used in a commercial environment are fantastic for home use, but are just additional complexity/points of failure. The service and support are where you will suffer.
2. Dell, HP, and others are far better at understanding all of the relevant specs necessary inside the PC for proper cooling,
3. Are you using SATA drives or SAS? SATA drives have less error reporting features and are more likely to shut down due to the lack of categories to slap errors into.
4. Xeon processors equivalent to the specs of an i7 are not much more. They are almost the exact same processor with some minor additional features enabled. The real value is they usually have a lower TDP and are overall a better/more reliable part due to binning.
5. The Windows error log might be your best friend on your current issue. That will help to identify if the issue is hardware, software, or OS related.
6. Windows embedded 7 is a good choice, particularly if you have your own build with unnecessary features turned off.
All lessons I've learned the hard way. The customer threshold for system instability is far lower in surveillance than in home/gaming PCs. I used to build DVRs/NVRs for use with Geovision to save a few hundred dollars per machine. We more than ate it up in support costs and I wasted many nights troubleshooting off hours to save the company money on techs resolving it. Now I just call Dell (through Avigilon) or HP (through BCD), get their tech onsite within 24 hours or less, and have a replacement part installed.
If you still want to pursue DIY NVRs, my recommendations would be to look for server grade boards from Intel, Supermicro, or similiar. At 128 cameras you are solidly out of the consumer grade component realm. The i7 is generally fine, but since there is such limited cost differential on an Xeon E3, may as well pay slightly more for top binning. Don't get the cheapest ram on the market as that will cause many issues that are difficult to identify. Verify all components are on the supported hardware list for the mobo.
I haven't had quite as good of luck with Seneca as Michael appears to have, but if you want reasonable pricing on homebrew PCs you don't have to support yourself, they are good option.