I bought a Stanley as well as four or five Zircon stud sensors. Truthfully, it's often a hit or miss deal with them, no matter how much you spend on one. The $20 Zircon at Home Depot is excellent and I rely upon that model rather than my $40 and my $50 Zircons which just sit in the truck. It's just as sensitive as its bigger brothers but smaller and lighter in weight and it has Deep Scan as well as AC hot wiring detect.
End Grain, I have two of the $20 Zircon's. One works great, the other, like you said is a hit or miss operation. And very frustrating at times. Now that I'm thinking about it, my life would be easier if I just popped it in the trash.