Hey Simon,

The way you're using PingPlotter is the best method for troubleshooting your issue.

It appeared from your post as though you had your suspicious devices figured out.

"I thought I had the suspect, unplugged it and everything returned to normal, plugged it in and things went haywire, but then everything cleared up and plugging and unplugging these connections made no difference."

It seems as though this device was an issue till you unplugged it, and returned to cause an issue when you plugged it back in. Everything clearing up afterwords is great but that device still caused those issues.

I recommend seeing if that device has firmware updates or if you can leave it off your network and see if it overall improves.

If you get some more data you want us to take a look at feel free to post it here.

Thank you,