[Dovecot] runaway imap processes

I am running dovecot (1.2.3) with postfix (2.6.3), postfixadmin (2.1.0), maia mailguard (1.0.2) and squirrelmail (1.4.19) on Linux (kernel 2.6.27.8).

I have an ongoing problem with imap processes that hang and eat all available cpu. I try killing them and they won't die. The only way to get rid of them is to reboot the server.

Any help with this will be greatly appreciated. I really need to get this solved as it has been an ongoing problem for more than a year.