Office 365 Outlook clients intermittently losing connection

I’m the one-man IT department for a small business. We use Office 365 Exchange with client machines all running Windows 7 Pro, and either Outlook 2013 or 2016 on each machine. Occasionally, (ranging from 0 to 5 times per working day, per machine) the Outlook clients will lose connection to Office 365 whilst still remaining connected to, well, everything else.

While this is happening, nslookup for outlook.office365.com will fail on whichever machine is currently experiencing the connection issues. The DNS requests will time out. Other machines on the same network (and joined to the same domain, using the same DNS server [we’re using active directory with a Server 2016 Essentials domain controller doing DNS locally]) will still be able to connect and nslookup will not fail on the other machines.

This usually lasts for 30 minutes to an hour before the connection is restored, or until the client machine is restarted, which instantly fixes the problem. Closing Outlook and re-opening does not fix the issue.

Any ideas here? I’ve been dealing with this for about a month now. Multiple support calls to Microsoft with about 10 hours total spent on the phone have gotten us nowhere on this issue. Support has screenshared with me while i’m remoted into the DC/DNS server. Still no fix.

Any ideas? I have tons more information/screenshots/debug outputs I can provide as needed.