It stopped as mysteriously as it began. Four days without an incident.

I did gather the associated transaction printouts and was getting ready to matrix those things together with processing times and digging into whatever logging and info I could get on my host's time-corresponded issues -- but the problem just stopped.

I'm going to guess it was network or server congestion, causing the returned zen cart page to timeout or error.

The only proactive action I can think of that might make an impact in the future is to increase the script timeout settings... maybe.

I might also instill with our cust service rep to gather more information about the customers if it starts again (browser, displayed error, etc..). Perhaps I'll make out a small template for her to email customers. A short questionnaire.

Is there anything else you can think of?

(Oh, and PS, I think I can relate your position on the Authorize/Akamai stuff. I've been trying to explain to people since the late 90's that cookies aren't viruses, but are pretty useful when you don't have a login over a stateless protocol. To this day people still ask about them and I get a little tense - I suspect from repeating myself for nearly two decades.)

Be well. And thanks so much for the attention. If I ever do find a definitive cause, I'll be sure to come back and fill in what blanks I can.