Establish User Endpoint is sometimes very slow

Hi,

I have a UCMA 3.0 application using user endpoints. The server is a Lync 2013 server.
Normally the "establish user endpoint" routine is executed in less than a second, but in periods this takes 22 seconds to execute.

This is the last line of code to execute before the "pause" :
userEndpoint.BeginEstablish(EndEndpointEstablish, userEndpoint);

Restart of the server where the UCMA application is running makes no difference. 
I have tried an older version of my application and the old version has the same problem so it seems as the problem is not related to changes in the UCMA application.

Any ideas where I can start looking?

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Jon R.