I have a column with checkbox .. if I scroll down while having some checked and some unchecked and up the check boxes start to check and unchecked randomly .. I tried to change them with a button labeled checked and unchecked still the same issue. Any help please.
Assuming you're using an itemRenderer, this is due to default itemRenderer behavior in Flex. The player calculates how many items will display at once on your screen (say, 10), and it creates that number of instances plus an extra buffer instance for the top and bottom of the list. When you scroll through your list, the player reuses existing instances, setting values on that instance from properties in your data source.
To fix this, have the check box set a boolean value in your data source. In your setter method in the item renderer itself, check that value and set the checkbox accordingly.
Adobe Developer Connection has a series of articles about itemRenderers if you want to learn more.