Just when I think I've got this PTO adventure figured out..........

Meeting with the principal today revealed some serious concerns about the behavior of our volunteer coordinator. We (board) know this person tends to be negative by nature but we've learned to laugh it off/convince her of the bright side, etc. What we didn't know is this behavior is leaking out into school activities and events. She represents the PTO and I need to find a way to change her behavior in a tactful way.

Here's some examples:
-group of teachers show up on their break to help plant flower bulbs. Afterwards they report they will never help again because she jsut b**ched the whole time
-a two year old visiting the book fair liked a pretty eraser and was walking around the gym with it. She yelled across the gym at the mother that if the child walked out with it, she was stealing
--BIG, BIG event she helped us plan. The night of the event her and her husband stood in the corner complaining about the music, food, wait times, etc
--confronted a student who was browsing at the book fair (in front of all his classmates) and accused him of stealing - "I know you took something, where is it"
--we reach consensus at EBoard meetings and then when we get to the general meeting she has more than once blindsided me by loudly and very negatively opposing the idea, project, etc.
--at a meeting a teacher expressed concern that some children may not be able to afford a field trip. She said, "Well isn't that too flippin bad. When you have kids this is what you sign up for."

She has several hundred volunteer hours. If you asked her to dance on top of the school she would. I would hate to lose her help. Our school is small and we are all very close and so are our kids. How do you tell someone "I value you. I love you. You make a difference. But shut the heck up?"