"Serius Network Trouble" error constanty, some modules down

Hi,

We are getting errors again with zulu. We get lots of lines like this in the GUI:

chan_sip.c:4274 _sipreliable_xmit: Serious Network Trouble; _sipxmit returns error for pkt data

After a time, zulu, xmmp, ucp and restapps daemons felt down. Help me please to solve this... We have about one hundred of extensions working with zulu.

Regards