the way the store order details works is to loop through each line item in the cart and perform an INSERT operation to the MySQL database.
If you have 600 line items in the cart, this will cause the loop to take a little while to complete all 600 inserts.
most likely this is causing the script to exceed the php Max Execution Time. this is a setting in the php.ini that limits the amount of time a php script can run for
you can try adding the following code at line 1 of the page with the store order details behavior on it to increase the max execution time setting to infinity:
<?php set_time_limit(0); ?>
it's also possible that the MySQL server has a limit on the number of INSERTS that can be processed in a certain time period, if that's the case, you will need to contact the host.
ass for overwriting the order, the store order summary behavior sets a session variable that is used as the OrderID if this session is not cleared after the order is stored, it will overwirte the previous order. Most likely when you tested the second time, you did not close the browser window first so the previous order session was still alive.