Is the 520 c also a sounder ?
Does the GPS cable (raymarine) and the sonar cable run close to each other through the trunking ?
Try using the lowrance alone with the raymarine switched off. If it works it then has someting to do with the above 2 wires.
I have a lowrance chartplotter and a huminbird gps/sounder. If I turn on the Lowrance I loose the gps signal to the huminbird
Tim