Stalls can happen at any point during a flight. They're less likely to occur once the plane hits cruising altitude, but it can happen. Remember when we discussed the plane jumping up to 40 000 feet? That could have put the plane into a stall, but then the plane would have crashed into the ocean at that point and it would have been found during the initial searches. I believe that happened shortly before the plane disappeared off radar.
Yes, having those black boxes would be a huge help! Until then, we're stuck here speculating :/
I'm still stuck on the Maldives sighting