Troubleshooting engine malfunction

My Peugeot 405 (2003) has become difficult to start. Fires and dies two or three times before able to idle. It has a significant flat-spot when accelerating from standstill. This is accompanied by by plume of whitish smoke. Behaves normally thereafter except that very occasionally the engine will die in full flight. Have to deploy hazards to make it safely to the side of the road. Trying to restart it does not work immediately. An intermission of +/- five minutes allows it to regain its composure and start normally.