I'd suggest you to check if your system memory is working correctly. Memtest86
is one of utilities you can use for it.
Thanks for the thought. I ran Memtest86 for 68 minutes with no errors; it got to about half way through test 8.
Just by the way, the biggest help was uncompressing the disk. That took me from +/- 95% failure to 75% success. I updated the Daemon Tools and its SPTD driver; that had no noticeable effect. I had already done driver updates and Windows patches. As we covered, the file system is fine and defrags with other tools. I've had the same failures in Safe Mode. I uninstalled all A/V software. There are no wonky behaviors I've noticed in any other programs...
So... I suppose I will start to look at rootkit infection, as I verified not much was running in Safe Mode other than Windows core processes. I could also try to find a BartPE ISO and check if I see failure that way, but that wouldn't seem likely or point in any particular direction either. Send me a link if someone knows where to find a bootable disc with the MyD installed.
But at this point, I've done a healthy bit of System Disk Monthly, 20% into zone 6 before I stopped it; and, I have all the performance boost I was looking for.
I'll keep this in mind, and post again if I learn something. As I've said, I agree with Jeroen that best guess is something weird with my system, whether Windows or the hardware.
Thanks for your help and your interest!