Periodic bad block errors

I have a six year old Ztsystems desktop with a Hitachi 1 TB internal. I periodically (once a month?) get a rash of bad block errors (64 in an hour, 562 in 24 hrs) which needless to say wipes out any disk access. CHKDSK comes back clean with 12 kb of previously marked sectors. Disk utilization is at only 15% of capacity and I periodically defragged the disk. I am running Windows 10/1803. Any suggestions would be greatly appreciated.