Gradebook

I'm having a weird problem with the gradebook. There are 70 students in the course and if I display all of them at once on a single page only the grades of the first 13 students are saved/updated and all the rest are left blank.

The workaround for the problem is to set the "students per page" setting under "my report preferences" to 13. That way only 13 records are displayed and all can be updated/saved. Since this solution is only a temporary fix which many of the teachers dislike and it hiders their grading I would like to get to the root of the problem. Turning off "quick grading" is not an option.

Anyone else with a similar problem or any ideas what I can check?

Thanks in advance,

Borut

Environment
Moodle 1.9.3+ (build 20081126), same problem with 1.9.2+
MySQL 5.0.51
PHP 5.2.5
All PHP extensions required for this version are installed.