Many thanks for that, Gary. Very helpful. Your machine sounds top-of-the-line so the problem can't be its performance. Just to confirm though: you are getting the message with that machine when running a quiz on this machine (coderunner.org.nz), even when you refresh the page? Because that's very strange indeed.
I could possibly come up with theories that would explain the problems on your own server but I'd first like to understand how problems can occur on our server.
I suggest we take this conversation off-line until we've found a solution. I'm going to email your directly.
Thanks for helping out