Presentation fading in and out on graph =S

[MichaelAngelo]

I haven't used my new Lowrance in shallow water yet, so I took it out for a spin today in 6 feet of water. I thought I would dead stick some minnows and test out the unit.

I noticed that every few minutes, the marks of my non-moving presentation would fade out, and then about 30s-1min later they would reappear...

This is a non-moving presentation in 6 feet of current-free water.

Any ideas why the marks are fading in and out every few minutes? My sensitivity is not on auto, it was set manually. Is the unit bad?