I know I am late on the scene, but the issue was not really solved and this may be of interest for other people passing by.
Indeed I have recently investigated the exact same issue:
-> it seems Dovecot does something unsupported by Via U2250 when installed with any flavor of Linux-64bit, ending up with recurrent system freezing and no log.
-> I experienced that situation on 2 Debian 7/64bit mail servers powered with U2250 (running Exim + Dovecot-imap).
There are 3 options:
-> find an alternate to Dovecot and keep the rest of your software and hardware configuration (alternates: uw imap, courier, cyrus - I achieved stability with courier but it was not convenient to me)
-> keep going with the same software solution but change hardware (Kikinovak's costly option)
-> change the running instruction set of your processor by installing an x86-32bit Linux (my choice - same packages selection, same config files and it works perfect)
Hope it will help.
Resource 1: two weeks of tests going back and forth between 3 64bit servers, 2 of them powered by U2250.
Resource 2: a bunch of people struggling with a U2250 config and Dovecot - in French: linuxfr.org/forums/general-general/posts/crashs-etranges-de-serveur
Resource 3: your thread.