In case it helps anyone else, the intermittent 'no sim' on a new Nexus 5 was in my case not due to software or firmware, but lateral movment of the sim card in the tray. I noticed many people who had this problem mentioned they used an adaptor (nano micro). I also do. A piece of sellotape across the back and folded over the shortest long edge (i.e. adjacent to the 45 deg notch) solved the problem perfectly.