I just found this from Garrett!
Good morning Ben. Thanks for being a Garrett customer and thanks for your patience . I certainly agree with you that $800.00 worth of product needs to be doing the job right ! I want to help you with that .That being said , it could just be your sensitivity is too high. I like that you tried another coil , that helps . Secondly , have you done a Factory Reset

It`s imperative to do a Factory Reset !when you change coils . Next , where is your sensitivity setting ? How is your coil cable wrapped ? These things really impact the performance of the detector. The coil cable should always !come over the top of the stem and wrapped snugly around it without any loose cable dangling above the coil that could create false signals. Since it is such a sensitive machine , if your sensitivity is too high , it will do just what you are describing . When you do a reset , it does 4 things . It automatically puts you in Std. Coins mode , puts you in F3 frequency , puts your sensitivity at TWO BARS BELOW full, and ground balances you at 80. These are optimal operating features and should be tried for testing your detector. All testing should be done outdoors away from outside electrical interference. Please try these things and see if it helps at all?*
Best regards,
!
Rusty Curry
Customer Service and Sales
Applications Advisor
Garrett Metal Detectors