I would be more concerned about why she is having these episodes...
Whether she's spooky, or she's attempting to move away from pain, or she's simply got your number and is doing her best to get out of work, it does not matter - the problem will persist regardless. It's just as dangerous to have a horse act out when attached to a harness and cart as it is under saddle.
Check her teeth, check her tack, check her back. If those are all fine, take a good, long look at her training and your leadership skills.
We drink to our youth, to the days come and gone; for the age of oppression is now nearly done.