Percidae Papermouth,
You say that you are seeing evenly spaced vertical lines. Is this only when running the other sonar unit on your boat? Try turning it off and see if that clears it up. If it does, turn the second sonar unit back on and adjust the Noise Filter menu in the 385ci unit until the sonar interference goes away.

If the interference does not go away when the other sonar unit is turned off than try turning everything else on the boat off as well (motors, livewells, radios…). Than start turning each device back on one at a time until the interference shows again. The noise filter menu may help with these as well but you may have a grounding issue on your boat like DTG is referring to.