I have a strange issue on one of the site. When I log in or register as a new customer and adding few items in shopping cart. Then go to the checkout page it is automatically logged out the customer in some random cases.
Sometimes it just logged out the user when I come to the checkout page. Sometimes it logged out the user when I go to the payment selection page. This is random behavior. Also in some cases, I can able to place an order with logged in but in success page, it is automatically logged out the customer.