Excessive Disk I/O by System (Process 4) for about 15 minutes several times a day

I have a computer with 3 rotating disk drives containing a total of 7 partitions, 16 GB memory, Windows 7 Professional 64-bit, and a fast 4-core hyper-threaded CPU.  Windows was installed 9 months ago, and gets updated automatically.  The issue is that, several times a day, "System" (process 4) starts doing massive disk I/O on every partition simultaneously for about 15 minutes.  Resource Monitor tells me that all the I/O is inside the System Volume Information folders; that the disks are 100% active; and that process 4 is doing 10 to 40 MB/second of reads and writes.  This I/O occurs on all partitions, including archival partitions that have not been accessed for several days.  I have disabled all the system restore points ("System protection") with no effect.  The I/O load is bad enough that programs trying to play music suffer noticeable dropouts.  The CPUs are not working hard (maybe 5% loaded).  I never had this trouble with with WinXP and a similar disk configuration.  Any idea what is causing this, and how I can stop it?