I don't know what your logfile look like, but you could try to look for thread IDs and try to discover some more information.
Try cross-referencing the access.log files with the info in your error.log or server.log files - since they are timestamped, you may be able to see cases where a request took a long time to serve; and you could then look for clues in that part of the logfile.
That may or may not be worth your time, however.
