Here is my input. I realize you have cleaned the pistol and magazine, but have you changed magazines? I have been a "Sig" guy for more than a few years, the P226 has been a mainstay of my gun safe since I was 21. In that time, I have never had the issues you are describing. That being said, I have heard of theses issues happening with just about every type of handgun out there. Kimber to Glock.
So, I would do two things. First, try different magazines. Next, take it to a local qualified gunsmith and have him/her look it over. Could be something small that is just missing your eyes. Perhaps a spring issue or extractor one. Next is back to Sig. Now, I believe that you mentioned that the accuracy issues have been in something that you have had happen with multiple Sigs. Ok, could be that for you, they just need some sight adjustment. I shoot fine with Sig, Glock, S&W all right out of the box, but Ruger I need to adjust. Thats just me.