Cars are a wonderful thing...

I have a 2001 Nissan Sentra with 208,000km on it and up until a couple weeks ago, it's been fairly low maintenance and I haven't had much trouble. I always get oil changes and do regular maintenance but I guess things can still go wrong with cars, right? The problem with my car is that it stalls. It might stall while I'm at an intersection waiting to turn left (very scary!), while I'll driving through town, while I'm in Park, while I'm braking for traffic (it's happened on hwy 400)--basically it happens a lot. I've taken it to a local mechanic, who my boyfriend trusts and has been going to for years, but he can't figure out what's wrong with it. He's checked the battery, alternator, mass airflow sensor, he's adjusted the RPM's because he thought it might be idling too low, he's done a tune-up, and cleaned multiple hoses and valves but nothing seems to be causing the stalling. He suggested taking it to Nissan and hooking it up to their diagnostic machine because he was stumped for ideas. Although I try to avoid dealerships I decided that I'd pay the $85 an hour to have them figure out what's wrong with it. They replaced the fuel filter because after talking with many people about the symptoms that seemed like the answer. Well, after $300 worth of work at the dealership, it still stalls. No one can figure out what's wrong with my car--not even the company that made it.

Does anyone have any suggestions as to what might be wrong with my car?