The persistent state of a link is just an Indigo level setting/preference on a link. It tells Indigo that you want the link to be re-written back to the device on sync if the link is missing. Links created in Indigo default to persistent. If the module is reset or swapped, then on the next sync Indigo will automatically re-write those links to the module. Links added externally to the module (manually; not from Indigo) arenot flagged as persistent by default.
Have you tried plugging the module into the same power strip as the PowerLinc? Can you add/sync it there? Also try plugging the PowerLinc into different circuits (different outlets). Since you are just troubleshooting you can do this quickly by just using an extension cord. Does it work better somewhere else? If so, then you know the problem is because of signal noise or attenuation and not because of a malfunctioning PowerLinc.