I recently bought a new Nox 600 and have had similar issues. After pinpointing especially, I would jusr lose the target altogether when returning to scan mode. This is in the woods away from power and homes, iron and lead/copper targets. I think Chase Goldman has some good pointers. I'm not a very experienced detectorist, at least not with a quality machine, and still learning the Nox 600. I'll also try a factory reset and turning off tracking ground balance as Nate suggested. I am in extremely rocky soil in the Catskills. "Two stone for every dirt" as they say. But I've been impressed by detecting items under a few inches of bluestone.