Originally Posted by OKCAg2002
One thing that has come to mind. I don't aerate very well. I usually just pour in the yeast after some shaking of the fermenter. Could that be a big problem?
You describe the off-flavor as being sour, which is usually indicative of bacterial contamination; however, it's also possible that you are detecting Acetaldehyde. Acetaldehyde is often described as a green-apple-like flavor, but I think it could very easily also be described as sour.
I also had a very bad Acetaldehyde problem in my first several extract beers. My usual solution was to let it age for a couple months, and eventually the off taste would simply go away.
At about the time I stopped using my secondary fermenter, the Acetaldehyde taste almost completely disappeared from my beers. I think I was unintentionally oxygenating my almost-finished beer. The flavor always seemed to intensify after bottling, just as you are describing.
More information about it can be found here