But soaking them isn't good enough. You can't clean and sanitize dirt. You gotta take those things apart and see if there is some gunk built up somewhere. When you are soaking it, you are only hitting the surface of the build-up. The nasties will live all throughout that and they will have a fertile breeding ground in-between brew sessions. As someone mentioned, you may be introducing a house yeast and a scenario such as this could be exactly how. You mentioned that you have had this problem on two rigs now. Can you recall when was the last time you had a successful batch? If I was you, I would not focus on parts of your process that have not changed. For example, if your carbonation process worked when you were brewing good beer, and you have not changed it, why would that suddenly be an issue? Since your process was nailed down previously, and you were making good beer, a more likely scenario is that another variable has introduced itself; hence the house strain theory.
