sys_messages table keeps crashing when cron runs

if ( $db_clean_msg > 0 ) {
            $res = db_res("DELETE FROM `sys_messages` WHERE `Date` < NOW() - INTERVAL $db_clean_msg DAY");
            if ( $res ) {
                $db_clean_msg_num = db_affected_rows();
                db_res("OPTIMIZE TABLE `sys_messages`");
            }
        }

I am wondering if the Optimize Table is causing this?

Any database people here?

Less likely OPTIMIZE TABLE cause table crashing, however you can comment out this line to see if this helps.

I have searched the net and looked at the server.  The error log reports nothing but the crashed table.  My config is:

query_cache_type = 1
query_cache_limit = 2M
query_cache_size = 200M
key_buffer_size = 64M
max_heap_table_size = 512M
tmp_table_size = 512M
thread_cache = 128
open_files_limit = 4096

Did you try to check mysql error log ?

Yes, of course.  The error log shows no errors reported until it reports the table is crashed. It is definitely crashing at cron job run, every night.  I would suspect the drive but it only crashes when the cron job runs.  It is not crashing during the day with active members on the site sending messages.  I may export the table, drop it, then import the table and see what happens.