Hi, hmseb.
Your crash interval seems quite regular.
Did you look into any external processes on your server? Like automatic backups, CRON scripts that might be running every 12 hours?
The rise in spike you describe could be a sign that some process isn't able to get past a certain point during script execution.
If you have a huge database (containing many old post-revisions or holding corrupt entries) a backup-script (or any other script that's working with the database on a regular basis) could be responsible for the crash.