Gopher, I agree with them about your settings. When I bought the M6 I was told to run and sens just a little about preset to hear everything. Well I did and it drove me goofy on one sight. I have in fact had to turn the sens down to about 3/4 running the 9" coil on the M6.
Last week I turned the sens down to 1/2 just to see how far down good signals on coins were. I got 5 solid clad quarter hits at 5" on a watered football field. These quarters at that depth were so pitted and corroded I couldn't read the dates on 3 of them. The other two were in the mid 90s. So even with these soil conditions the M6 did great at even 1/2 sens and really reduced the chatter for me.
Try setting down your sensitivity like the others suggested and see it that helps. Oh the other hand, maybe your M6 is reading more and smaller targets then your friends machine. If that doesn't work, maybe your coil is bad or the connection is loose. Let us know what you find out.