Electrical interference

hello, wondering if anyone has any ideas of cause and how to fix? 

have recently installed a 1kw tranny on my head unit. 

Previously had two different types of 600w trannys with no issues whatsoever. 

the issue is that I get a single line of what I think is electrical interference when I kick the motor into gear from neutral, and sometimes immediately when just starting to accelerate from a constant speed eg moving from 10 to 15kts quickly.

The interference is only very minor eg a single vertical line from top to bottom of screen, or smaller vertical line mid water or even below sea bottom. It's only intermittent...somtimes just one fine line or a couple of fine lines in the time it takes for a full screen scroll/refresh, then it goes away. Very rarely it also happens at cruise at a constant speed but not often.

not a massive deal and the increase in picture quality from the bigger tranny more than makes up for it, BUT it gives me the Sh!ts no knowing what causing it or how to fix it!

as I said never happened in the past, only since installing the 1kw tranny. 

head unit is wired into batteries with no common wiring shared with anything else.

i was thinking a couple of potential causes, wondering what others think??

-increase in revs causes alternator to put out more into battery and that spike shows on screen?

-increase in revs/amps causes interference with the 1kw sounding?  

-interference between sounder cable and other electrical cables in conduit (but didn't occur with other tannys)?

-batteries on way out and increase in power draw from new tranny is causing minor spike when alternator increases output (happens on both batteries thou even when completely isolated from each other, so don't think so)?

-other cause?