I’m not the best at diagnosing network issues.
Yes, in 99% of the cases, you think the problem is the network and it’s not. But in rare cases it actually is.
A friend of mine has written an awesome guide to diagnose network issues using traceroute. He also happens to have founded a startup named Okio doing network monitoring.
Read it, I have learned tons of things from it.
Full disclosure: In case of doubt, I have no interest in this company. I do think they have a great product and that the guide is a fabulous piece of work.