Problems with Skype and Lync calls through GC108

Skype & Lync calls work fine in the following environment:

However, when I introduce a GS108 switch into the environment:

I experience drop outs of up to 5 seconds about every 30 seconds during Skype and Lync calls. I can also see by the lights on the router that the connection from the GS108 to the router is temporarily lost during the drop out period.

Any ideas what's happening and how to fix it?

Thanks,

Adrian