When it goes to the cart page, does it show the cart as being empty?
Usually when this happens it a problem with domain switching and sessions.
what i mean is changing from using www in the address to not using www
take not of the domain in the address bar when you see this happen.
If you access the site using:
add some items to the cart and view the cart at:
then take the www out of the address:
you will see what i mean.
A different session cookie is created for www.mydomain.com and another for mydomain.com
they way to get around this is to force all traffic to go to www.mydomain.com using an htaccess rule: