1997 Olds 88...sporadic stalling issues

Bought my daughter a 1997 Olds 88. Ran and drove great when i took it for a ride before buying it (Saturday). Same thing when I picked it up and brought it home for her 2 days later (Monday). Registered the next morning (Tuesday) and she took it out that bight with her friends. She called and told me it died. When I got there it was hard to start. It would crank but not start. After 6-8 tries, I held the gas pedal down and it started, drove it home with no issues (only a few blocks back to the house). She told me that she while she was driving it, it started to sputter and then finally stalled and wouldn't start for her. Ran fine Wednesday but I didn't trust it for her to use it while my wife went away for a few days. We returned on Saturday and it ran fine. Sunday & Monday was the same. Tuesday I took it to work with no issues (8 miles round trip). One daughter went to Starbucks and home with no issues (also about 8 round trip). Then to school and back that evening ( again less than 10 miles round trip).Tuesday night it happened again with the first daughter. I went and it started on the second try...also while i gave it some gas. But the Check Engine light was on. Acceleration seemed to "bog down" a little, but then caught. Never stalled during acceleration or at stop lights. Got code P0171. I changed the Mass Air Flow sensor and hope this works. I changed it last evening and then drove the car around town, start & stop like a teenaged daughter would do, and I had zero issues. Acceleration was fine. But she just called and sed it started fine, but stalled when she lowered the power window?? She hasn't gotten home from work yet, so I haven't been able to drive it yet. IDK what to try next if this doesn't work.