Thinking of string life, I used use Ernie Ball Earthwood stings on my old Yamaha as I liked the sound that combination seemed to give. I actually hated the initial brightness of the strings but found that with some hard playing, that would wear off in a matter of hours. I was then left with strings that I liked the sound of and they seemed to stay consistent for a long while. The main reason I had for changing a set was they reached a point where I could no longer tune them properly.

What causes that - they can sound right say on the second fret but sound miles out on the fifth. Do parts of the strings become thinner than others with stretching?

Jon