I doubt that this is about the speed of your server or the volume your processing. At Kuroi we have who clients whose sites peak at over 100 orders an hour and we've never seen the problem you're describing, let alone with a 10% error rate.

Each customer's order details should be held in a separate php "session" and if the server were overloaded all that should happen is that everything slows down and grinds to a halt, not that information from one visitor's session becomes available to another.

This sounds to me like a candidate for problems with how php has been configured on your server, rather than the Zen Cart application code.

But you'd need somebody with more expertise in that area than me, to comment further. It might help though if you could post some information about your server, php version, and how php is configured to run on the server.