"The specified network name is no longer available." intermittently

(I took a look at a few other similar questions, but they don't seem to be about the same scenario that I'm having)

I have trouble with some systems at a remote office, connected via site to site VPN sometimes having issues accessing Windows shares.

User Bob at the remote facility connects to a share on the file server at the central office, sees the data he is after, and goes to copy it, but it hangs for a moment and he gets an error "The specified network name is no longer available."

User Jim also at the remote facility can still hit the share on the file server, and copy data, no problem.

User Bob can ping the file server successfully. User bob can hit UNC shares on other servers at the central office. (however....)

As a workaround/test I copied the data he needed from its location on the file server to a folder on another server at the central office, and shared the folder. Bob could hit that UNC path, and went to copy the data and suddenly gets the same error.

After he gets this error on a system, he can ping that system, but attempting to access its root UNC path \fileserver or \otherserver it gives Error code 0x80070035 The network path was not found. However at this time he can still hit UNC on other servers at the central office.

Server is running Windows Server 2012. Bob is on a laptop running Windows 7 x64. Another user in that remote office has reported sometimes having the same issue, but not at the same time as Bob. Meanwhile Jim continues to have no issues, and can copy the data down for Bob when we can't access the server. Bob and Jim are in the same active directory groups.

Nothing in event log on the workstation or on the server.

On the workstation I've tried disabling and re-enabling network interfaces, wired and wireless. Updated network and system drivers. Rebooting sometimes fixes it, sometimes not.

This one has me stumped. Thanks for any help!