Make sure you have a stable version of both PM and the SSBB NTSC ISO. I don't remember the whole rigmarole to finalize the installation via Dolphin, but there are some shitty versions of both items out there that can really mess things up.
Also, make sure you don't move the build around when it's running, it hates that. Final thought, idk about for brawl, but I know Melee is actually sensitive to the storage size of the SD card when it's run off of SDs. Without seeing HOW it's crashing, I have no way of diagnosing it better. I am sorry, friend.