melius2 wrote:It looks like a network issue, maybe a NAT problem, have you tried changing Apache default port from 80 to other to see if you can replicate the issue?
Ok, so for all the sites I work on, I set a subdomain of my company's domain (i.e. acmepaint.mycompany.com) to point to my public IP, which my router then NATs to my local development workstation which is running XAMPP.
For whatever reason I decided to try adding a hosts file override to map those subdomains to 127.0.0.1, and viola! The problem disappeared instantly. I'm not sure if people outside of my office are still having intermittent issues, but I don't really care as that's a secondary concern for me.
So, thanks for the suggestion, which led me to the above idea. This is (seemingly) the end of a year+ nightmare that has made my development work absolutely miserable.