A few weeks ago I took my F5G to have some setup work done on it. The action was terribly high and I was having trouble with the A-strings not holding tune. The luthier thought maybe the nut was pinching one of the strings, but he said it looked fine and he said the keys looked fine. But I'm still having trouble with the A's. I can get them in tune and as soon as I start playing, one of them will go flat or somethimes go sharp. I can't figure it out. If it's not the nut or the key, what else could it be?