Form validation problem

I have a form for data entry and use validators to validate some of the fields. I am having a problem with one part of this form. I have a series of checkboxes (checkboxlist) that when a user clicks on one it populates a readonly textbox with a value. I also have another seperate checkbox that also adds a value to that same textbox. I have a requiredfieldvalidator that validates the textbox. That functionality seems to work fine until I try to submit the form. If I check any of the checkboxes in the checkbox list the form submits. If I only check the other stand-alone checkbox the textbox clears and the requiredfieldvalidator fires and tells me the textbox is required. I have no idea what is causing the textbox to clear when I hit submit and why it doesn't happen when one of the checkbox list checkboxes is checked and why it does when only the stand-alone checkbox is checked. I will paste the html and code behind belo ...