Before you run PingPlotter or speed tests, do this!
How's that for a clickbait headline? But this is really about that headline.
When you're experiencing problems with Cox internet service, you need to check and confirm the reliability of the RF network before you do anything else. The RF network is the network that runs on the coaxial cable that comes out of the back of your modem. Problems in the RF network are very common, and if you don't fix those, results from PingPlotter or speed tests aren't valid. Sometimes RF network problems are caused by your equipment or your wiring, but more often these problems are in the Cox network and are outside of your ability to control.
Most modems will let you check signal levels and event logs by going to the IP address 192.168.100.1 in a browser. You need to find out how to do this for your individual modem. You can't skip this step. The RF network is the foundation of everything. Trying to troubleshoot an issue without checking the RF network is like building a house without a foundation.
There is information available on many sites about modem signals and most of it is accurate. If you post cut/paste signals and modem logs people on the forum will often help you understand them. The only bad information I've seen out there are some Arris pages that make people think their upstream power level is too low. There's basically no such thing as upstream power being too low (unless it's zero) so ignore that. Upstream power being too high is definitely a thing and indicates problems in the RF network.
The most important indicator on the downstream signal page is uncorrected codewords. If you see uncorrected codewords continuously increasing, there is a problem in the RF network. That needs to be resolved before you run PingPlotter or speed tests. Most times you will need a Cox tech to fix this. Every now and then you might get a burst of a few thousand uncorrectables overnight when Cox does maintenance. But if any uncorrectable numbers are increasing during the day, you have a problem that PingPlotter and speed tests won't fix.
When you check the logs, look for CM-STATUS messages (minor issues but if you see them happening more and more often, get ready for bigger problems), dynamic range window violations (minor upstream RF issue but usually indicates that worse problems are coming in the future), T3 timeouts (indicating a bad problem on the upstream RF connection), T4 timeouts (indicating a very bad problem with the whole RF connection), and unexpected modem reboots (usually caused by the modem rebooting to try to re-establish a signal on the RF network).
Once you have good downstream and upstream signals, virtually zero uncorrectable codewords, and clean modem logs with no T3/T4 timeouts, only then should you move on to running PingPlotter and speed tests. Otherwise you're just wasting your time on a problem that those tools can't identify.
TL;DR: Don't waste time with PingPlotter and speed tests until your modem signals and modem logs are clean and correct.