I have a hefe in the keg now, which is at 15 psi at about 37 degrees. When I pour it, it looks perfectly carbonated. The beer itself, not the head. I am getting quite a bit of sour at the end of the taste. If I let it sit for about 10 min, when it looks like it had about half the carbonation, the sour is less. When the beer is just about flat it tastes great, just no carb. Any ideas?