Car that has recently broken down stalling while idling

Hi! My car broke down earlier this summer on the interstate. The engine was smoking, and the steering locked so my dad and I had to put it in neutral and push it to safety, and get it towed. The serpentine belt had given out, and in the shop they also replaced the A/C compression and vacuum. They said it was safe to drive, but in the last month it has stalled while idle on 3 separate occasions. Each time has been within the first few minutes of driving, at a red light or stop sign. The engine stops, so I turn the ignition to off, back on, and then continue without further problems for weeks. Other than this, the car has been driving smoothly since the work in the shop two or so months ago, including making more than one longer drivers (between 2 and 5 hours). What could be malfunctioning? Could it be related to the problems from earlier in the summer, or something different entirely? Are there any particular warning signs I should be looking for for a proper diagnosis?