I was once puzzled several months by my web server with an issue. It SOMETIMES can not load even a mid-sized files although the small files can work pretty well. It is so frustrating as some of my friends can read my files while others claim they can not.
It takes me long time to figure out why. Lots of research and experiments. I exposed my web server outside of my filewall, reinstalled web server, reset modem/router, … none of them seems to help. All of a suden one day, I figured out it is caused by an MTU issue. It should be 1492 for DSL in router settings. After I reset MTU value correctly in router, server works perfect.
If your web server sometimes work and sometimes not, it is more likely an MTU issue.
I’ll detail this later, if I have more time and if I can still remember all the details.