Golden biting when tired or needs to poop

Question
We have a 7.5 month old female golden who was spade about 4 weeks ago.  About 8 weeks ago she started biting and snarling pretty aggressively at least 2x per day.  Yesterday it was about 4x.  There doesn't seem to be anything she or we are doing that triggers it.  She could be playing with us or by herself, resting, or out on a walk and will just suddenly start lunging at us, biting repeatedly.  If we crate her she does go to sleep almost immediately.  We have been told this could be FRAP, or she probably isn't getting enough exercise, a trainer even told us it is "youthful exuberance"  We have tried exercising her more often and for longer periods of time but it hasn't helped.She hasn't done any real damage to any of us yet, but has scraped us up pretty badly.  What could be causing this and what can we do to prevent it?

Answer
Spaying her was a good move.  Certainly she wasn't a candidate for breeding.  That trainer sounds like the reason I am reluctant to suggest professional trainers.

The key to most behavior problems is approaching things using the dog's natural instincts.  Dogs see all the people and dogs in the household as a pack with each having their own rank in the pack and a top dog.  Life is much easier if the 2 legged pack members outrank the 4 legged ones.  You can learn to play the role of top dog by reading some books or going to a good obedience class. A good obedience class or book is about you being top dog, not about rewarding standard commands with a treat. Start at http://www.dogsbestfriend.com/  For more on being top dog, see http://www.dogbreedinfo.com./topdogrules.htm