If you changed the key-shell, you would probably recall moving the little black plastic thing - the chip, which is actually responsible for the start (SKREEM reads the data of that chip using the plastic ring around ignition). I think your chip is not held where it supposed to be. I have not seen this happening, but I was told that this used to be an issue on W140s. When the chip "jumps" backwards, the ring does not read the data, and the vehicle does not recognize the key. It actually can go into a temporary start error stage, which is clears by itself soon as the vehicle "sees" the correct chip. Perhaps try to shake the key and see if you hear the chip jumping. I must say that it sounds more like an RCM issue. If you will ever develop an actual start error - contact me, I will fix it for you. 9366666650