Have you over/under-clocked or otherwise adjusted your CPU settings? This "solved" issue which seems to be similar is put down to processor core instability under certain conditions:
https://bbs.archlinux.org/viewtopic.php?id=290093 As the last comment mentions, I was also wondering about the possibility of thermal issues. Might a smartmon or memtest test be worthwhile? You don't seem to be dumping from read-only snapshots afaics, but that these particular files might be changing between checksum and compression (or whichever comes first) seems an unlikely spanner in the works. HTH Gareth
