I know nothing of this model metal detector. The aperture models I know have a transmitter coil and two reciever coils. The receiver coils are wired out of phase so they cancel each other for a near zero signal. As something conductive or reactive passes through, there's an imbalance in the reciever coils and some level of signal to detect.
Anyway, many problems can be from the mounting framework. Attachment points within the magnetic field must be securely connected or securely isolated. A loose connection in surrounding framework that touches and opens with vibration causes problems. Current loops through the framework making and breaking interfere with the detector. Where practical, we have a single mounting point grounded and the rest isolated. Where there is a loop, make sure it doesn’t intermittently open. Also avoid metal movement near the detector. Make sure guides and chutes are secure with minimal or no movement and are securely isolated or securely grounded.
Another potential problem for us is drifting values of aging components that balance the reciever coils. That would be factory or factory trained service.