How did you measure the resistance of all 4 plugs?
Try to clean the plate that connects all the plugs, maybe you have a bad contacts here.
But anyway, at +15 engine should start easy, even you have no plugs at all. I would check the injectors. If injector doesn't spray properly (leaks), the cold engine can't burn the fuel. When temperature rises the engine is able to burn anything the injectors supply. It seems similar to your condition.