I'm sorry to hear that maliaros.
How does the AT Pro act around your house (back yard)?
Something isn't right maliaros. Are you in detecting mode after you manual ground balance? You have to hit the button again after you GB manually.
Just some suggestions here:
1- Walk outside with the MD (and away from any interference)
2-Turn it on
3-Hold the coil in the air about 3'
4-Push the power button in and hold it for at least 5 seconds (2 beeps for factory Reset).
5-Reset the MD for Pro- Zero-No Discrimination-No Iron Audio
6-Bring "Sensitivity" down to 1-Bar showing (Low)
7-Auto Ground Balance (only) slowly 5 or 6 times then release the GB button in a clean area
8-Make certain your in Auto Ground Balance first Not Manual (Displayed on screen)
9-Then swing the coil just above the ground and slowly bring up the "Sensitivity" until it starts to make noise
10-You can keep it their or bring it back down one notch to make it a bit quieter.
Do this procedure (6-10) over different areas other than the beach for now and see how the MD reacts to different conditions. If you get the exact same results, then something isn't acting the way it should.
-Bad Connection
-Bad Battery/Batteries
-Bad MD (simple as that)
This is the way I narrow things down to Outside conditions affecting the MD or the condition of the MD itself, this is what I do every time if I get any interference with my MD. It's only happened a couple of times it was always under ground power cables affecting it.
When I can get a hit on a drain pipe which is (11/2 - 2' in dia.) that's 36" deep with a good VDI (have the video to prove it) then you can see how much more sensitive the AT Pro is from your 350. Having said all of that maliaros, it's important to enjoy this great hobby of ours and if means switching back to something you feel more comfortable with, then that's what you should do then.
I'm trying to "Exhaust" all avenues here before you make that decision.
Let us know how things work out or what you have decided maliaros, thanks.