I've been tuning guitars for a long time, but never stopped to ask anyone about this. When using an electronic tuner I often find that the note varies from dead-on to slightly sharp or flat over the period of time I let the string ring. I also sometimes notice that the string will ring sharp or flat depending on how hard I play it. Maybe someone can explain to me why this is happening, if there is something wrong with my guitar set-ups (intonation???), and if it is normal, is there a best way to manage this flip-flopping pitch to get the best tune? Thanks!