Builds Stuck in Queue

Hi,

We are running 8.1.3 (30101) of source, upgrading from 8.1.1 about a month or so ago. It was going well for a while, but in the past week or so, the builds are getting stuck in the queue.
I have not been able to find the cause, and the only thing that has made a difference is restarting the teamcity server - but this only lasts about a day, before the queue stops moving again.
I have attached the logs for if anyone can see something I have missed.


Any help would be greatly appreciated.

Cheers,

Mark.