I have to respectfully disagree with you spirithorse, My 7 yo gelding liked to buck when you asked for canter...He was just full of himself. He was young and a bit green and when you asked for a faster gear he got goofy.
I'm going to suggest you check for pain and other issues that the rest have already suggested and then perhaps seek out a trainer, this could be something as simple as her saying, Ahh, no, I'm not going to work today.
If she's always had this issue, (previous owners had it too) It may just be a gap in her training. I bet a good trainer could have this quirk worked out in a couple weeks, surely less than 30 days. Money well spent IMO.
Good Luck and Be careful!!!
Keep us posted on her!