Slave is started in Nimbula cloud - our own plugin is used to start virtual machine. Plugin is not published yet.
But I really don't think trouble is in it.

Steps are:
1. Job is started
2. Our plugin is used to start VM to run job
3. VM starts
4. It connects on start to Jnlp url
5. Build process starts
And.. sometimes ~1 of 10 starts (rather rare) build process hangs.
Console is in such state:
Started by timer
Building remotely on Nimbula-slave-1110 in workspace /jenkins-slave/workspace/PRODUCTION - Sanity
[Throbber non stop here]

Node system info is in attachment.
I would be very much obliged if you can help to understand possible reasons!