Stall right after ignition
 

My wife and I have a 2007 9-3 2.0 with 39M miles on it and we've enjoyed the car a bunch. Lately, about every 4 weeks or so, she takes a bit more turning over before firing and then once she fires, she stumbles and then dies. The car will start right up immediately afterward and run just fine. It has done it once on warm evenings more or less, but the last time, it was the morning after a cool front came through and drop temps from the 70s to low 50s. Any thoughts? Vacuum issues, maybe?