I didn't read all of the other responses, but scanned through a few and it looks like you got a lot of good information already. Hopfully I'm not repeating too much, but do want to stress that some other posts already mentioned: What you described sounds like more of an involentary things, not that she is purposely holding her breath for attention. And to me, the fact that it seems involentary and she seems scared by it worries me a lot more. Talk to your doctor, or a second doctor again - and make sure you tell them she is not just holding her breath, but that she CAN'T get a breath, and mention the seizure like activity. I'm not trying to scare you, and more than likely it is nothing serious that she might just grow out of. But it does seem like something that should at least be looked into.
I am a nurse, but don't work much with kids and don't have specific experience with this. But this is something as a mom and a nurse that I would want to make sure I had ruled out any deeper problems. Hope this helps a little, good luck!
S.