Where exactly have you declared the variables?

My best guess is that you're declaring "j_currentPrincipalBalanceRef" before the element with the ID "txtCurrentPrincipalBalance" is drawn to the screen. The reason it would work in the function is that the function is being called after that element is drawn, whereas when you declare the variable at the global scope, it gets called sequentially.

Try declaring those variables at the bottom of your page in a script tag, rather than in the header.