I'm pretty sure it's because you've blocked off the blow-off valve, as your pushing it hard, the AFM sense's more air going past it, than when you let off the accelerator to change gears the engine thinks there's more air than there is (AFM assumes that it's running a plumbed back BOV), injects more fuel and stalls.. Atleast that's my thoughts, common on people who put atmo bov's on stock ECU's..
Are you running stock ECU? Try getting an aftermarket ECU, or plumb back in your BOV.. surprised search didn't find it.. SAFCII has ability to stop this from happening, same with any standalone ECU..