Have you tried taking the pH as the beer matures to confirm that your tastes are correct?Sadly I think this batch is also turning sour too. It seems to have lasted longer than others before it was detectable but I'm definitely noticing an increase in acidity. I'm at a bit of a loss really now, I've totally stripped everything down on the last brew and kept things simple with as little messing as possible. I guess the only thing I can think of is the CO2, but I've got 2 bottles from 2 different suppliers and I'm pretty sure I've had the sour issue when using both bottles.
One interesting thing, I just hooked up the full keg to see if there is a difference between that and the keg I've been using and I would say I can taste the sourness on both, but more so on the one with less in that I've been using.