Training a Pony

Question
We recently purchased a POA that is 6 yrs old. My Daughter is riding her and
we have had her for 4 weeks. We went to our first show of the season this
past weekend and the pony showed us her ugly side. We our not strangers to
the pony world and know that they can be little devils. The problem we are
having is this: She rides really nice at the barn we have her at. It's a really nice
indoor arena. She would act up every once an awhile but my daughter would
take control and keep her going. BUT..... at the show she would pop up go for
a little bit then stop suddenly and start pranching around and popping up
some more making it very difficult to get one lap around the arena. This
would happen mostly by the gate! My daughter lunged the pony before we
went to the show. We did one class of pony haulter did really good and got
first place. But when it came time to ride she just really showed herself. So my
daughter lunged her again. Got back on and still had problems so we
srcatched the rest of the show. Not sure which way to go at this point. We
have Fair for 4-h in 4 weeks and would really like to get this under control so
my daughter will have a 4-h project. Also the people we got her from said
that they have shown her so she isn't new to the show seen. She does not trail
ride very well and the previous owners haven't rode her since November of
07. Any advice would be greatly appreciated.

Answer
Your pony is known as "barn sour", more specifically, "gate sour".  You will have to be persistent and refuse to give in to your pony to overcome this.  I have pulled three articles I think you could benefit from reading.  Read each of them and if you still have questions about it, please ask me.  I will be happy to help you understand how this may have happened, and how to apply the articles' techniques to your training.  
 1. Monty Roberts Answers: How do you cure a barn sour horse?
    http://www.montyroberts.com/ju_ask_monty_1104.html#CureSourHorse

 2. Marv Walker - The Ring Sour Horse
    http://marvwalker.com/ringsour.htm

 3. Julie Goodnight - The Gate Sour Horse
    http://www.juliegoodnight.com/questionsNew.php?id=73