Javascript Checkbutton help

Hi,
I'm having some trouble with a portion of code I am writing. The code works the first time round, but then becomes unresponsive on one part.

What I am trying to have is the following...

Enter a value in the input field, check box gets ticked.
Untick checkbox. clears value in the input field
Tick checkbox , sets a value in the input field
Untick checkbox. clears value in the input field

Enter a value in the input field, check box gets ticked....

Javascript Checkbutton help WordPress ...