I'm leaning towards Corte's suggestion of the ECU, due to the intermittency (you say it runs normally at times before stalling).

The smoke is just a good way to tell what's going on. If it's a fuel issue, you can generally see it in the exhaust. If it's a "my brain isn't telling my engine to run" issue, the exhaust tends to look normal when it does run.

Edit: I wouldn't go out just for the purpose of checking what your exhaust looks like. I was just asking in case you noticed.