First I've heard of items replacing one another in the download cart! Um, I've really got nothing.... Perhaps check your security / firewall system to make sure that you're allowing all types of cookies? The download cart relies 100% on cookies to retain what you have in your cart. If it's replacing your stuff every time you add something, then it's got to be erasing the cookie and putting another one on.
Maybe try adding something, then go to view your cart (you can click on the same gray Add to Cart button again and it'll take you to your cart), then go back to where you were (just click Continue Shopping) and add another item.
Sorry I don't have more suggestions, but this is the first I've heard of this. Just make sure your cookies are unrestricted.
Stay messy, my friends.