Metal Detector Messed Up : /

Can someone please help me here? I have a brand new Fisher f-5 that I purchased about a month ago. Almost every time I take it out on a hunt, about a half hour or so into it, it starts to go nuts. There is just "chattering" as you might call it. It still picks up items but it gets so confusing because the discrimination is bouncing from iron all the way to quarter simultaneously...It gives me completely false readings. I constantly would change my frequency to the 3 different presets in case that a power line or whatever was interfering but it gives no sign of difference. I would even put my threshold down to elimnate some signals but nothing. Can someone help me out here? I have no idea why it randomly starts to wack out like this...thanks.