You seem to be trouble shooting it in a logical way.

I think I'll run this by one of my network guys. It may be a problem with the network but I'm not sure. How big is the network and what kind of switches do you have?

What procedures have you taken to separate the voice from the data? Have you used VLans? When did you purchase and install the system?

Let me check the 7.1.8.1 software. I want to see what the next couple of versions corrected. The 6x is on 8.1.9.4 right now.
They are still supporting and upgrading the software although the replacement (the Connect Series) has been out for about a year now.