It's impossible to troubleshoot an issue that cannot be reproduced.
It's possible is could be a server issue, it's also possible it could be a myriad of other things.
if you could reproduce the problem, we could probably figure it out pretty quickly.
examine the email receipts that are sent for the orders that fail to insert to the database, is there a common item in the cart when it fails? it could be one of the product descriptions causing the insert to fail.