If you turn off the 999 and the issue goes away ...it's most likely crosstalk interference cause by the 200kHz sonar beams overlapping ...
You can turn on and up the Noise Filter under the Sonar tab ... Or ...
If your 798 is indeed the HD model ...you can Ethernet the units together for both head units to operate from 1 xducer to eliminate crosstalk ...
Or you turn off the sonar on one unit (short press the power button) ...and use this unit as a Chartplotter only ...
Rickie