I have had a problem with beer going bad after a week or two in the keg. I think my problem was inadequate cleaning and rinsing of Oxyclean. I have switched to PBW and I am slowly getting away from this problem. The rinsing is easier and my equipment is becoming squeaky clean again.
The last batch in question was 10g and split into 2 corny kegs. One keg turned bad and the stayed very good. I marked the bad keg and cleaned them both with the same PBW method. I rinsed well and filled them both with 5g of Starsan of the proper concentration. They sat this way for 2-3 weeks.
Today, I was preparing to rack a fresh 10g batch to these kegs, and for no real reason, I racked the Starsan to white plastic buckets. GUESS WHAT? The "bad" keg Starsan was cloudy and the good keg was clear. Both had pH of 2.5 so the Starsan was still good.
I think have heard that hard water (minerals) can cause Starsan to get cloudy. I am on city water which is relatively low in mineral content. My guess is that there was some mineral deposit, maybe beerstone or leftover cleaner residue, that turned the Starsan cloudy.
Thanks in advance!
Side Note: I did not use the "bad" keg. It is outside soaking in PBW again. I racked to a different "good" keg.