Don't worry about it.
I have had two different ACE 350's.
Sometimes this happens to me and is quite annoying, but I figured out the problems for me: It was electrical interference around where I live/and or a few too many trash targets in the yard. If you go into a yard with quite a bit of trash in the ground, even little pieces, the coil is seeing 11" at a time in a straight line down the middle of the coil. It, therefore, sees multiple targets at nearly the same time. and will "Chatter". Try volleyball courts, beaches which are not too trashy as some may be, and sandy areas, especially tot lots. Go a little slow, and make sure to try it in different areas, not just where you had problems before. This has totally solved my problems. I realized it is not the perfect coil, and under certain conditions, it is almost non-usable. I got an extra 11" DD from a friend who bought one for his ACE250 and couldn't use it because he tried it in the urban area with too much trash in the ground and electrical interference (DD's suffer from more sensitivity to that). He barely used it, and was disappointed.
On the plus side, in a sand Tot Lot, the coil easily found me very tiny pieces of various metal targets, plus my first two gold targets ever (one tiny and hard to find)!
Overall, remember: It is "Area Specific" that determines if you will have a problem or not, and when you don't, this coil is killer sensitive on anything and goes deep.
P.S. The typical advice to "Lower the sensitivity" is NOT the solution :P. Most trashy areas still have too many targets for the 11" DD search area even near the surface of the earth. So, unless you lower the sensitivity to 1" searching, (who wants to do that!) it is not a coil to use in trashy areas. For that, I use my ACE 250 6X9, with way less problems, and I can set the sensitivity way higher than the 11", so it goes deeper and finds more than the 11" -- same with air tests around areas with computer WiFi and power lines: the 6X9 coil beats it because 11" must have Sens. down to keep it from constant chatter.
