I'm borrowing this from another poster in another thread, because it explained my point better than I could:
"gifted kids have what Debrowski called an "intellectual supersensitivity" in the 1950's. Basically, this means that gifted kids crave, and need intellectual stimulation like they need food, water and sleep. If they don't get it, they become behavior problems, failures to achieve, delinquent or sick (depressed, anxious, etc.)"
Just something else to look into, because the answer might be SPD, understimulation, or a combination of both. Because you mentioned how far ahead your DD is, and you mentioned she's not in the G/T program yet, so it sounds like she's not getting any kind of intellectual stimulation at school.
Plus, my own DD7 has been having the same issue to varying degrees, and her home meltdown frequency/intensity is inversely proportional to the amount of intellectual stimulation she receives at school.
