Adding to and Downloading Your Cart
I've worked with the folks who support the Gallery 2 software and have made some server tweaks to help with the aborted downloads.
The key is to keep your cart manageable. Adding multiple albums to your cart WILL result in a failed download at some point. If you must take everything, then do it one album at a time.
From this point on, anyone mentioning problems downloading their carts will be referred to this post. If you don't believe this solves your situation, then make a forum post with as much detail as possible, and I'll look into it.