I got a GTP 1350 not long ago and was getting pretty much the same as you. Rarely found anything deeper than about 3 to 4 inches. I did some searching and reading in this and other forums and found an interesting post. I'm not sure if this will work for you but it sure worked for me.
1. The origional instructions say to wrap the cable over the stem all the up to the box. Instead, strap in down (with tape or velcro (I found some velcro straps that work great)) straight up the lower stem. At about mid way up you can wrap it around.
2. The 1350 has a "threshold" setting. At first I thought this was an audio threshold but now I'm convinced it a depth threshold. It has to be just barely audible in the head phones. (the lower the better)
3. Set the sensitivity up all the way. If it really unstable at that point, back it off a notch until it's stable.
Like I said, I don't know if it will work for you but after doing these things, the same week I pulled out a 1926 D Merc and a 1947 D Quarter. The quarter was 6 inches down and the Merc was 8 inches.
On another note: I was born and raised in the Charleston, WV area. Nice to see someone from home on here!