Ok, so I do this about once every 2 or 3 years: actually take my car into the local mechanic with the lift. He diagnosed an exhaust system that was hung a bit too high, sagging motor mounts/trans mount and slight contact with the exhaust system. I'd replaced the trans mount a few years ago myself, the motor mounts are original (335k) and there still seems to be very little vibration from the engine. After some adjustments, he reduced the metallic chipping noise substantially, and just charged me for the lift time.

The theory is, this sound (which was almost identical to spark knock btw) caused the knock sensor to retard substantially under load enough to cause a stall. I've now had the car back on the road for a better part of a day, putting it through all the motions, and it's much smoother and responsive - and no stalling. Amazing what some metal on metal contact will do, thankfully not inside the engine - which was my worst fear.

Thanks to everyone for their suggestions. Those theorizing about exhaust/engine metal/lower extremities were definitely on the right track...