Using zen 1.3.7

During the checkout process, there is a button to click to change the billing address (also another button to change the shipping address. All of the comments/problems below are the same for either page).

When the user clicks this button, they are taken to the checkout_payment_address page. So far, so good.

On that page, they are given the choice of entering a new address or selecting a radio button for one of their pre-existing optional addresses.

So, here's my problem:
-when the user enters a new address, and clicks continue, a java screen pops up and warns that they forget to enter gender, name, address, state, etc...basically, the system thinks they have entered nothing in any of the fields.

-when the user does not enter a new address, but instead decides to select one of the radio button choices, then clicks continue, the same java screen pops up telling them that they have not entered any info.

-Now, here's the weird part...when the user enters something in the new address fields (total garbage such as all x's is ok), and selects a radio button address choice, then clicks enter...then the system is happy and accepts the radio button choice. The x's entered in the new address boxes seem to be ignored.

I don't need/deserve a long explanation, since I know that this is something that I have screwed up myself. But any tips on which files to check would be appreciated.

I have already checked the following and compared (winmerged) them to another install of zencart that is working perfecting with the same templates, but not found any differences or problems in them:

tpl_modules_checkout_new_address.php
tpl_checkout_shippingaddress_default.php
includes/modules/pages/checkout_payment_address/all files
includes/form_check.js.php

Thanks in advance for any ideas.