When I try to add multiple packages, any where from about 6-10, the cart will clear itself. I'm wondering if this has to do with the extra processing a package has to go through since the package information is saved as an array and then serialized? Is it just overloading the server causing it to drop the cart?
one of the support engineers has taken a look at this but cannot replicate your issue with our test install.
If you have not already, can you update Cartthrob to the latest version and if you still have the issue, please schedule a support call at http://support.mightybigrobot.com