One frequently mentioned fix for this is to replace the fuel injector relay. It is behind the fuze box. Some sensors can also cause this if they fail, but will typically set a code (e.g. cam position sensor).

As an intermediate problem, it can be very difficult to trace. My car was doing this, but I had a cam position sensor code set (I assume codes have been checked and you have none). I replaced both the cam position sensor (because the code was set) and the fuel injection relay (because this is frequently mentioned as a intermittent stalling problem), and it hasn't happened since. The fuel injector relay can be disassembled and the switch closed to place it in an "always on" condition, but this is a safety concern. If your car does it at regular intervals (i.e. daily) rather than every couple of weeks, you could try this and see if it stops. Mine would sometimes go for weeks without an occurance, so I didn't think it would be prudent to test in this manner.

Phantom