Order Number in Confirmation & AuthorizeNet

I have 2 challenges with my confirm.php page and hope someone can give me some ideas:

Everything works perfect except 2 things:

1.) The order confirmation e-mail that the customer receives does sometimes have the order number included but most of the time not (it is displayed on the checkout_success page but not in the confirmation e-mail).

I do have the code for the order number in the confirmation e-mail as follows:

<?php echo ((isset($_SESSION["eCart1_OrderID"]))?$_SESSION["eCart1_OrderID"]:"")?>

And as mentioned. It sometimes works but unfortunately not all the time. I thought that this would maybe be a browser compatibility problem but was not able to see a difference among all browsers.

2.) The Order/Invoice number that is transmitted and then displayed in AuthorizeNet is different from the customers order number

I have the following value in the server behavior for authorize net for x_invoice_num which is:

<?php echo session_id(); ?>

It seems to be different from the actual order number.

Any ideas for those 2 challenges would be appreciated.

Best,
Joerg