Hey badger,
You seem to be on the right track, the results you've got here look an *awful* lot like bandwidth saturation. We've got an example in our Common Network Problems article that looks fairly similar to what you've got here:
https://www.pingplotter.com/common-network-problems/#bandwidthsatWe've got a short video that goes over some tactics for analyzing PingPlotter results to figure out where a problem may be occurring (and we use bandwidth saturation as the example scenario). Have a look here:
https://youtu.be/3Raj7tX0TUQIf you'd like, feel free to send over a PP2 file (or use the "File" -> "Share" option in PIngPlotter and shoot over the link) - we'd be happy to take a closer look at your results and offer any additional guidance we can!
Best wishes,
-Gary