Check your gas lines! I'm working through an issue right now where everything was turning funky (mainly noticeable in IPA's or hoppy beers) and not good after a few days to a week in the keg. I've narrowed it down (I think) to crud or mold in the gas lines. I disassembled my lines and soaked them for a day in Oxiclean, rinsed, and let hang dry for a couple days. Just reassembled and kegged a nice IPA the other day. So far, so good. The gas itself is fine, but if your gas lines have mold in them from back pressure or liquid/moisture getting in the lines (pressure in keg higher than what's going in), you'll get mold build up and the co2 going into the keg will taint your beer. I'm preeeeetty sure this is what was happening to me. If it isn't, then I am absolutely lost as to where to look next; I've cleaned, sanitized, replaced hoses, started fermenting in kegs, doing closed transfers, make sure I pitch a lot of yeast, aerate well, the works!