I did extensive diagnostics with
Spinrite and WD Diag on each drive and
they picked up NO surface issues, no
sector errors, and no SMART warnings.
That's a pretty impressive amount of troubleshooting that I have to admit, after all that I'd be astonished to think anything was wrong with the hard drives. But after reading your post a little further, I think I found the problem.
Intel ICH8R motherboard controller
Now since you're going for speed rather than data redundancy I can see why using the on-board controller seems appealing but in reality nearly all on-board RAID controllers (especially for consumer grade motherboards) are crap. Highpoint, Intel, nVidia... all crap.
To Rik's point about power, that's actually a good point. Fluctuations in the power can have an averse affect on computers in general but also the hard drives. It might be easier & cheaper to use a UPS (uninteruppted power supply) for your computer to deal with the power issue.
Are my drives okay? Can there be
something unhealthy with one of my
drives that the diags are not picking
Since you run RAID 0, I'd say there's always a risk of something going wrong. Good thing you have a backup image elsewhere. I'd have to say though I doubt anything is wrong with your drives. Running Spinrite, WDDiag and looking for SMART info is pretty thorough. In all likelyhood, I'd blame the on-board controller. I've run software RAID, on-board controller RAID (both years ago) and now hardware RAID and I can without a doubt say that software and on-board were ultimately a complete waste of my time. I can't speak specifically to RAID 0, but if I had to guess what the issue was, I'd look to the controller.
If money is not an issue, I'd say get a hardware RAID controller in addition to a UPS. 2 port RAID controllers aren't too expensive and ironcially enough, I never run RAID 0 so I can't even attest to how a better RAID controller (from 3Ware, Areca, LSI, Adaptec, etc.) would do but I'm more certain that a PCIe RAID controller from one of the manufacturers I listed would be less likely to randomly corrupt your stripped array.