If Lync 2010 or 2013 have intermitent connection problems , this can be  caused by a misconfiguration on your SIP on your Lync Server or an Active Directory misconfiguration.

A workaround is to manually specify the Internal Server Name and the External Server Name with these values on :

Internal Server Name : sipdir.online.lync.com:443

External Server Name : sipdir.online.lync.com:443