I had the same thing happen to me after replacing an older b&w Lowrance with a LMS522c. Every time I stepped on the tm the sonar returns would drop out leaving a blank or garbled line scrolling across the screen as long as the tm was activated.
I recall correcting it by some adjustment with the sensitivity settings. It involved just changing the settings from manual over to auto or vice versa.
“There is no difference between communism and socialism, except in the means of achieving the same ultimate end: communism proposes to enslave men by force, socialism—by vote. It is merely the difference between murder and suicide.” Ayn Rand