In one of our Moodle classes, clicking on the "Grades" link results in a HTTP Error 500.

Censored Apache logs show:

PHP Fatal error: Maximum execution time of 30 seconds exceeded in moodle/lib/grade/grade_category.php on line 920, referer: http://.../course/view.php?id=3191

This happened once before a few weeks ago in a different course. Backing up and restoring the class resulted in a working grade book in the restored course. I'm willing to try some troubleshooting advice that anyone might have before doing the backup/restore workaround.

Setup:
CentOS 6.3
PHP 5.3.3
APC cache
MySQL 5.1.61 (CentOS/yum version) using MyISAM engine