Chris, thanks. The WPEngine folks claim they’ve scoured the Apache/PHP logs. The theme/plugin incompatibility possibility is one we haven’t yet systematically explored as we have lot. And, since the problem is intermittent, the sequence of steps is nothing special: we just bring up the page and sometimes the mashup shows, at other times it says “Loading…” and never loads.
Oh well, I’ll keep in touch if further clues.
Regards,
Jim P.