Thanks for pointing that out. The code on the confirm page seems to be out of order. I can see the store order summary occurs on the page before the transaction does. The transaction looks to be triggered by 'any page post', I think this should be 'same page post' instead.
I'm not sure if this is the reason for what you are experiencing but I think this should be updated before we try to troubleshoot the session variables any further. You should make sure to update this page and put the store order summary server behavior down further on the page just above the store order details server behavior. You may want to make a backup and test it at this point. If the result seems the same you should try updating the trigger for the local checkout server behavior to be same page post and test it out again.
Post back and let us know if there is a change in the result. From what I can tell looking at your page both the session id and order id session variable should be set after this page is processed. If the session issue is intermittent it may be much harder to troubleshoot.
