For what it is worth. I think I found the true culprit. I submitted my finding and suggested correction to http://www.zen-cart.com/forum/showthread.php?p=414755
Hopefully this will address you issues. From what I can see this fix alone has completely solved the problem for me (so I believe you can disregard my earlier suggested fix).
... and please don't flog me if I am beating a dead horse..
Good luck!






