Ml320 Dies At Idle 2nd Time

[DEPGOODY]

Last night after driving 30 minutes at varying speeds the Black Beauty died. The other day it backfired before doing this. When I try to restart it makes a funny whirring noise and I must turn the key left to stop. 10 minutes later the vehicle starts like nothing is wrong. The dealer changed the air flow meter but it died again 2 days later. During the previous 2 days it wasn't driven more than 15 minutes at a time. What could this be ?
Some component locking up at idle? Timing chain or belt tensioner? Please help! Thanks, Danny