Sound Drops

I am having an issue. I have all grandstream hardware. I have 6 locations that have a dedicated FreePBX server. All work great except 1. All devices are on a 1000mb HP Procurve layer 2 managed switches. All switches show no error on any interfaces. But when you are talking randomly the voice will cut out. The call will not drop it is just that neither side can talk/hear. If you are talking you may not notice but the other person can’t hear you. I see nother in the full log when this happens. Any suggestions. I am running out of ideas.