Why does my 2001 Monte Carlo SS (3.8 L V-6) stall?

My car will stall at traffic lights, intersections, or while driving around town (25 MPH), with no warning, no "check engine light", nothing. The car will sometimes re-start after 10 or 15 minutes but more often than not, I've had to have the car towed. Then, of course, it will re-start the next day. I've had the car to the Chevy dealer and they are stumped but tried cleaning the throttle body & throttle plates. Helped for 6 days but the car is back to stalling. Any ideas?