I recently migrated my company's intranet site from IIS on Windows 2003 to Apache on Ubuntu 14.04 and having been having a strange issue: once in a while, users get "This page can't be displayed" in Internet Explorer when trying to access it. I can't determine any rhyme or reason to why it sometimes fails like this. Interestingly, if the user clicks Refresh the site loads properly right away. This happens maybe 3 or 4 times a day (overall, not per user) out of hundreds of page requests and doesn't seem to happen to multiple users at the same time.

My issue is that without being able to recreate it myself, I'm having a hard time isolating the cause of the problem. This question's accepted answer is exactly what my first thought is - set up Wireshark and check the traffic. However, this issue is very intermittent and doesn't always happen for the same user or workstation, so short of having it run constantly on a large number of workstations, I wouldn't know how to ensure I can capture the data I want. At this point, I don't know if it's a workstation issue, webserver issue, DNS issue, or something else altogether.

Is there a good way to narrow down what the source of the problem might be? Is there anything I should be checking that I'm missing?

Additional Info

I don't have any errors correlating to this on my DNS servers (and I see the A records and they look good), but I do have a curious recurring error in the Apache logs -

[(datetime)] [core:error] [pid 17100] [client (IP of Domain Controller/DNS server):3753] AH00135: Invalid method in request  

When I google that error, it looks like the log usually lists the request with the invalid method, such as in this question or this one. In my case, it doesn't have anything at all after the Invalid method in request. There's roughly one entry an hour (including hours we are closed and employees wouldn't be accessing the site), always with the IP of my primary DC/DNS server (running Win2003). Not sure if a blank request is being sent to Apache or what. I'm thinking this isn't related, but don't know for sure.

If relevant, the workstations are all Windows 7 machines running IE10.

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