Comment by Diederich

I don't have time to type it up more completely, but back in the early to mid 90s I administrated, among others, some AT&T 3B2 servers. ( https://en.wikipedia.org/wiki/3B_series_computers )

There was an odd situation where some of the systems were unable to make connections to other systems ....some largish distance away. It was fairly variable, but some systems were almost always unreachable, and some were occasionally reachable.

Long investigation, but in summary, this happened because the default packet TTL was, for some reason, set to a fairly low value in a minor kernel update. I simply increased that number in the kernel, recompiled, and all of the problems went away.