I’m in desperate need of help. I have a bizarre problem that I’ve had no success in solving. See below the pertinent backstory and details.
This all began about two months ago. Two months ago the network consisted of two computers used 8 hours/day, and three more used much more seldom on a 12Mbps internet connection that splits the connection equally among clients. Basical mail/web browsing. No video streaming to speak of. We do have 5 Cisco VOIP phones, but they're used somewhat infrequently.
Since then, we’ve added a server/domain controller, and three additional computers used 8 hours/day. The three new computers were purchased from Dell Business, and all are running Win 10 Pro. No networking hardware changes have been made (router/switch). The network was then switched over to a domain network, with all computers enrolled in the domain except two personal computers running Win 10 Home, which are no longer used as of now.
DISCOVERY OF ISSUE
We didn’t get all of the new Dell machines at once. First we just bought one. Within a week the user was reporting the inability to send/receive emails in Outlook on occasion. When I troubleshot the issue I noticed the computer couldn’t connect to the IMAP or SMTP server, or the webmail login, or the server’s IP address. Couldn’t connect in Outlook, couldn’t connect in-browser to the webmail login, couldn’t ping the server in CMD or powershell, nothing. But a reboot instantly solved the issue. I tried setting up a different IMAP account in Outlook, but the issue didn’t occur with that account (GSuite Business account).
I tried disabling the firewall, adding the IP to the whitelist. No joy. Finally, I reinstalled the network adapter driver – nothing. Then I setup the account in Windows Mail – though it happened less frequently, it still happened. A reboot instantly fixed it, but I couldn’t find the cause. I even called our mail provider, thinking perhaps the machine was being temporarily blocked, but they claimed that wasn’t possible. Nothing in Event Viewer pertaining to the issue either. Nearing the end of my rope with this issue, I opted to reinstall Windows before troubleshooting further. After a complete reinstall the issue returned. None of the other computers experience the issue – at that time.
Shortly thereafter we purchased two more Dell PCs. Different models from the first and one-another. Both have the same issue, but the issue has also expanded. It’s not just the mail server that we can’t connect to after a time, it’s also websites like McMaster Carr, Yahoo Mail – etc. It almost seems as though it’s any site or server that’s frequently used/accessed on the machine in question. At first I thought it was an O/S issue, but we have another Win10 Pro machine on-site that doesn’t have this issue. We also have several Win10 Home machines (personal computers owned by employees) that don’t have this issue, plus two Win 7 Pro machines, also without this issue. It’s only on the new Dell PCs running Win 10 Pro.
TROUBLESHOOTING EFFORTS & RESULTS
I maintain this network remotely, so booting into Safe Mode with Networking to troubleshoot further isn’t possible, but it’s the next thing I’m going to try when I’m on-site later this week. The issue doesn’t seem to occur at set intervals. Sometimes things work great for hours and hours, then stop, other times 10 minutes after booting up the issue begins. I also want to wait for the issue to occur, then try to connect the computer to a completely different network. Haven’t been able to try this yet as only one of the machines has a wireless adapter, but when I’m on-site my goal is to wait for the issue to occur, the connect it to a WiFi hotspot on my phone and see if the issue persists.
One other potentially important note – the internet at this business isn’t DSL or cable. Those services aren’t available at this location; despite the fact the business isn’t in a particularly remote area. The service we have is a fixed-wireless system provided by the City. It’s slow and buggy. For example, I’ve been trying to setup a new router for 4 weeks, but they’ve been unable to get the new router that we purchased to connect. I RMAed the first one, thinking it was a router issue. The new router can resolve hostnames to IPs, but can’t connect to the internet or ping IPs. Very weird. We’re using Cisco hardware right now, but I’m thinking about switching to Ubiquiti depending on how this shakes out. Just need to make sure Ubiquiti routers provide L2TP VPN.
List of what I’ve tried:
Disabling all Dell OEM Software
Adding server IP & host names to allow list in Firewall
Add server IP & host names to whitelist
Reinstalling network adapter drivers
Reinstalling Windows (OEM edition – I don’t have another free license)
Setting up an IMAP Gmail account in Outlook and constantly hitting send/receive for 30 minutes
List of what I plant to try when I'm next on-site this week
Safe mode with networking to see if issue presents in this diagnostic mode
Change networks without reboot during time the issue is occurring
Swap router & switch
EDIT: ADDITIONAL DETAILS PER COWN'S REQUEST:
When this problem occurs access is only disabled to the site's/servers in question. We do NOT lose internet access on the machines. I can ping the DC and the router, and other machines. Other websites/servers can still be accessed without issue.
The problem doesn't appear to be caused by software or drivers, are three affected computers are running completely different software. The only common threads are: Dell machines, Win 10 Pro.
We're using a Cisco C881 router and Cisco SG300-38P Switch. No other machines on the network apart from the new Dells have this issue.
DHCP is provided by our Windows A/D DC. DHCP is disabled on our router.