CenturyLink is not only the worst ISP in the world, but is indisputably the worst provider of any service in human history.
300-500ms latency spikes at all times. Always over 50ms to the first hop (usually several times that.) Even with ancient technology this is not realistically possible. The only explanation is that their NIDs and hubs are nests of baling wire.
CenturyLink's only goal is to be useless at everything at all times.
There is obviously nothing they can do to remedy the "problem"; the problem is that they exist.
Chapter 11 was written for this company.