Problem has finally been solved after days of troubleshooting and head banging.   The practice has a Watchguard Firebox Firewall.  Part of my troubleshooting was turning off the PC AV and firewall and then disabling the practice firewall.  None of this helped.  After much persistence, it turns out, this particular firewall packet scans for for viruses coming into the practice at the gateway level.  I turned a setting from scan all packets to scan random packets and that seems to have resolved the issue!  Thank you all for your comments.