N20 engine shutdown at idle. No codes.

My x3 with a N20 engine is shuting off at idle (idles at 800 RPM then jumps to 1200 and dies. Starts right back up smoothl) so I took it to BMW with no codes! 140,000 miles. New fuel pump, wire harness inspection, coils and spark plugs all installed by BMW. Car just stop running when at an idle and no lights come on dash (ie no oil light etc) (start/stop overridden). I need ideas for my problem. Thanks Dave