lots of people have this exact problem. Have a look at the stick thread "THIS LOW RPM ENGINE STALLING SAGA POSSIBLY SORTED"
I have heard of people wiring the pump to run at full capacity all the time, turning up the idle speed, cleaning the AFM/AAC's and many more other solutions. Some people on skylines downunder even suggest that it is air reversion back through the AFM.
My car had the exact same problem and I have virtually changed the car back to stock and it still happens (but very rarely). I changed:
Turbosmart BOV Type III to Stock
Blitz pod to Stock air box with K&N air filter
Replaced the tiny battery with something more powerful
Changed all vacuum hoses
Adjusted the idle speed to about 850 rpm
Cleaned the AFM
My car rarely stalls and I'm happy with it, except the noisy lifters but I am going to fix that.
I have found that a stall is more likely to happen when I have my lights and lots of electrical gizmos going