engine is shutting down while driving and at stop signs

I have Ford Taurus 2007. Last few months, it would stall when I stop at red light or when I was slowing down and cruising. It would turn on without problem immediately. Also, I would feel a kick and everything would shut down for a split second when I was driving about 40 mph. It happened just couple times. My certified mechanic could not find the problem and ended up replacing some relay for $30 because it was a primary suspect. However, the problem did not disappear. I took car to the dealership, and they said I need to replace camshaft sensor and synchronizer as well as ignition coil (total of $800). My mechanic says that they are simply guessing and he truly doubts that both things are problems at the same time. He also says that their diagnosis makes no sense to him. I don't know what to do?...