Sounds to me like a disk problem. We recently had a bunch of failed
200GB SATA Maxtors with 16MB cache in a RAID array. The Maxtors
seemed very sensitive to ambient temperature (we suspected the disks
had been temperature stressed although client clains an ambient room
temperature of 75F), were purchased in Jan of this year. Install
bonnie++ and run it on the disk without encryption and with just
reiserfs, see if the disk dies or you have error messages. The
non-dead maxtors in the array died with between 4 and 30 hours of
stress testing in a room with an ambient of 65F... the conclusion
was, that once stressed, the disks are on their way to failure, it
was just a question of time.