Occasional Page Not Found errors when adding to cart.

Randomly, when trying to add a product to the shopping cart my users are redirected to the zen cart page not found page: http://www.atomic-ranch.com/store/index.php?main_page=page_not_found

I've read through some other posts that deal with what seems to be a very similar issue:

• http://www.zen-cart.com/showthread.php?196736-Getting-redirected-to-Page-not-Found-when-adding-to-shopping-cart/
• http://www.zen-cart.com/showthread.php?42387-Sporadic-Add-to-Cart-Button-doesn-t-Add

I've temporarily added some code suggested in the first link above and collected some possibly useful info:

http://pastebin.com/RtjdXbCD

I've tried it with & without this .htaccess file at the root of /store/

# replace /yourstorefolder/ with the name of the folder your store is located in, if any.
# be sure to add any extra paths required to locate the index.php
# then relocate this file to the root of your store's folders, and rename it as .htaccess

ErrorDocument 404 /store/index.php?main_page=page_not_found

I've tested the setting for "Admin->Configuration->My Store->Display Cart After Adding Product?" with both true and false settings.

I tried "Admin->Configuration->MyStore->Missing Page Check" with all 3 options:

1. off - Adding product to cart still occasionally redirects to page_not_found but visiting an invalid page gives me a partially rendered page as expected.
2. on - Adding product to cart still occasionally redirects to page_not_found but visiting an invalid page redirects to the store index page as expected.
3. Page Not Found - works as expected

I inspected page_parse_time.log but didn't see anything interesting:
http://pastebin.com/tLUgsr7c

I inspected query_selects_page_not_found*****.txt but don't understand what it says:
http://pastebin.com/Rj0k8Mn9

What else can I do to solve this mystery?

thanks,
Josh

You could try my "Email on Page Not Found" plugin (http://www.zen-cart.com/downloads.php?do=file&id=1090).

Thanks, I'll give it a try.

Tried but all I get is:

A user at host address [snip] with IP address
[snip] generated a 404 using the query string
[/store/index.php?main_page=page_not_found]

Any other suggestions?

I've noticed if the url includes 'www' then it works 100% of the time.

But without the 'www' it works 0% of the time. At least now I have some direction.