Issues with user.sub files - leaves imapd running at 99 % of processor

[Adam D]

I have noticed this error in the log:
Sep 12 18:57:44 vts-post cyrus/imap[19629]: IOERROR: locking
/var/lib/cyrus/domain/W/domain1/user/A/userbox.sub: Interrupted system call

Soon afterwards I notice imapd runs at 99.8 % and I can not kill the
process.  It looks like the file is locked and can't be written to?  I
saw another file called userbox.sub.NEW but even that files can't be
read.  When I attempt the terminal freezes.  I also noticed the system
can't be rebooted as well.  I don't know what could be causing this. 
Does anyone have any incite? 

Example:
-rw------- 1 cyrus mail  31K Sep 12 18:22 user.seen
-rw------- 1 cyrus mail  14K May 24 10:21 user.sub
-rw------- 1 cyrus mail 4.0K Sep 11 18:25 user.sub.NEW

I can view user.sub but it has not been updated since May and the
user.sub.NEW apears as an binary file or a DB


  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+ 
COMMAND                                 
19484 cyrus     25   0  154m 4772 3776 R 99.8  0.8   7:51.32 imapd     

-Adam