Hi Chris,
Have a look at this code which you can add to your wp-config.php file.
It will log any and all php errors to a file, independently of whether you have WP_DEBUG set to true. That way you can keep track of PHP issues on a live website.
I'd be very interested to hear if any errors relating to this issue show up in the log.
Be aware: logging errors will generally scare the living daylights out of most people as they suddenly start realising how many plugins are throwing errors....
Hope this helps!
Smile,
Juliette