2001 Bonneville! Several weeks ago, had trouble turning the key in the ignition - sometimes would turn, sometimes wouldn't. Then the Service Engine Soon light came on and the car stalled out a few times while driving. Then one day last week, the key would not turn for several days. Had the ignition changed out with a certified GM part made for our VIN and the Service Engine Soon went out for 1 day and car ran fine. Next day, Service Engine Soon came back on and the car is again stalling - more so now than before. Key works fine now. Any ideas where I should begin troubleshooting?

SOURCE: engine intermitantly stalls

if this a fuel injected car just check to fuel pressure first.

SOURCE: service engine soon light is on code reads f.airflow censer

take back to dealer. you paid for a solution not a guess. demand they fix it because they should have done it right the first time. remember you paid to have it fixed not have parts changed on it at your expense for no reason.

SOURCE: Possible security light problem on 1999 pontiac firebird...

if you have the chip in the key, then this will surely help you: http://newrockies.com/vatsbypass . It was the only thing that saved my car.

SOURCE: Periodic stall issue. All guages go to zero,

Sounds like an intermittent short in the wire harness or ECU(computer).
If you can locate the computer(in the passenger side kickpanel), try
tapping on it with a plastic handled screwdriver handle, to see if the car
suddenly shuts down? If it does, your computer is shot. Good luck.

SOURCE: Car just stalled while driving.

sounds to me like you have a short in the ignition itself , it seems intermittent which would make me believe its the ignition