Purchase and Purchase confirmation don't work in IE and Safari

hello, hoping someone has a miracle up their sleeve.

I have been browser testing confirmations on www.golfcartking.com. They are having trouble as some users are not getting purchase confirmation and for those users purchases are not getting collected in the system/paypal.

SETUP:
---------------------------------

- URL: http://www.golfcartking.com
- ZenCart Version 1.3.9d
- Host: HostGator -- ZenCart installed from CPanel. No upgrade.
- Using PayPal Express/Pro for payment

ADDONS
---------------------------------
- SEO URL Addon
- Drop Ship Module Addon


BROWSER TESTING/ISSUES:
I tested the process of purchasing with a new account and an existing account to see if I could pinpoint when a user would/would not receive confirmation/payment -- found some discrepancies. Hoping someone has a silver bullet for the Safari and IE issues below.

---------------------------------

CHROME/OPERA/FIREFOX: purchasing w/ NEW account
- customer created
- received confirmation screen
- welcome email
- received confirmation email
- payment went through

CHROME/OPERA/FIREFOX: purchasing w/ EXISTING account
- received confirmation screen
- payment went through
- received confirmation email

---------------------------------

SAFARI PC: purchasing w/ NEW account
- customer created
- received confirmation screen
- welcome email
- NO confirmation email
- payment went through

SAFARI PC: purchasing w/ EXISTING account
- received confirmation screen
- payment went through
- received confirmation email

---------------------------------

IE 7/8: purchasing w/ NEW account
- customer created
- welcome email received
- NO confirmation screen
- NO confirmation email
- NO payment went through

IE 7/8: purchasing w/ EXISTING account
- NO confirmation screen
- NO payment went through

Anybody have any ideas why it would work in Opera, Firefox, and Chrome -- but not IE and Safari? Makes it a little hard to troubleshoot.

I sincerely appreciate any help in advance!

This might not have anything to do with your problem, but ...

IE can be very persnickety when presented with pages with validation errors, and your main and login (http://validator.w3.org/check?verbos...n_page%3Dlogin) pages have errors so I'm assuming that there might be issues with your checkout-related pages, too. Fix those validation errors first, then see if you still have a problem.

lat9 -- that appeared to be the ticket!!! THANK YOU!