this happens often on that model.
-when the screen goes a little whack, it's just adjusting to your auto depth range.
- when you hear a beep but nothings there 1 of 2 things are happening.
1. you see a blob on the screen. i've seen alot of these usually marking a school of baitfish. (if you want a good way to test this toss out a very tiny lure, and see what you pull up, i've nailed perch and shiners all kinds when this was happening. )
2. your over a very weedy flat bottom and there's a fish burried underneath all the crud.