Another thing that could be possibly involved is intermittent failing of finding Plex on the local network, or it not picking up the authentication which will make the stream loop back over the internet. How do you have PKC configured? Is it using the same method os the official add-on or are you using the option that drops in local links to the library instead?
As long as your playing direct this shouldn’t be an issue if you set it up with path substitution. If you look at your log you will see when you are direct playing it opens the same as any other Kodi file. Path substitution in Kodi modifies any path when it gets a match. Therefore you just need to setup a system mount (autofs or fstab probably makes little difference) and check what path is being opened and manipulate it accordingly.