Could a cron job be running on a different machine that is accessing this one, a backup process or some other IT thing?
Are you running stats? This can cause spikes. And if the stats are not finishing correctly they can start running at times other than the scheduled time. Our stats keep creeping back 5 min. But it is tied to a cron job, so your no cron at 6:14 seems to contraindicate. Look in the Moodle docs for stats and: