Basic History on redundant servers
Do you have any third party backup (eg Acronis) performing hot backup? Disable it and see if it helps (ABB has *NOT* certified any hot disk image backup tool).
Do you have large amount of logs + large amount of secondary aggregated logs? Try disabling the secondaries - there has been situations when aggregation has run out of memory, etc. when log template has many levels of secondaries.
Does Windows Performance Monitor (not Task Manager) show any increase in AdvHtHistorySrv.exe -> Virtual Bytes or Physical Disk -> Average Disk Queue Length over this 24 hour period?