IMAP stops working after upgrade

[zyxxyz]

Server stops(i mean it stops work with user queries, service is working according to zmcontrol stats) working 1-2 times a day, there is no any errors in logs, accept messages about "sqltrace - Slow execution", and cpu usage grows but not more than 30%.
When this happens, i see that almost whole system begin work very slowly.

Zimbra is running on VirtualBox with 8Gb of Ram and 4 CPUs

Situation is similar to one described in thread

That things begin after upgrade server to UBUNTU 10LTS and to Release 7.1.1_GA_3196.UBUNTU10_64.
I made all configuration according to Performance Tuning Guidelines for Large Deployments but it changed nothing.

[zyxxyz]

Todays morning a new message appeared in terminal:

task java:blocked for more than 120 seconds

and server stop working at all nether ssh nor even keyboard...