General help

We are running a moodle installation on Linux in an FE college with 1000 students, with about 40 - 100 students accessing it at any one time. 

More details in my colleague's post:
http://moodle.org/mod/forum/discuss.php?d=31308

When mysql hangs this query seems to be the culprit.  Are there any coders out there who could suggest where in moodle this occurs and what activity might be causing it to run?

SELECT DISTINCT u.id, u.id
                              FROM mdl_user as u,
                                   mdl_message as m,
                                   mdl_message_read as mr,
                                   mdl_message_contacts as mc
                             WHERE m.useridfrom = u.id
                                OR m.useridto = u.id
                                OR mr.useridfrom = u.id
                                OR mr.useridto = u.id
                                OR mc.userid = u.id
                                OR mc.contactid = u.id

Thanks