Hi robert,
The issue you mentioned is css issue.As you are using Ajax control toolkit,you have to override existing bultin css.Fixed the issue with the following css definition:
#ctl00_CR_TabContainer_TabPanelDetails_tbText_ctl02_ctl00
{
height: auto !important;
}
The selector is page specific but it can be determined easily using Firebug.
Hope this will help.