There is normally code on the checkout pages that will check to see if there are any items in the cart, if not you will be redirected back to the cart page. You say more often than not that this page does not work correctly. Have you noticed any pattern to it, does it always do this if a particular item is purchased? Also, is your checkout working with https pages only? Please post back with these details and a copy of your checkout success page so that we can take a look.