Did not read the whole 4 pages, but:
- Any slow DB queries, or intensive queries that are being executed together? Enable the slow query log and check what will happen. If there are slow queries that mark the start of the overloads, are they malformed queries or are they regular ones? If they are a result of the overload and not the cause, they would pop up after the CPU spikes, so then what is PHP doing at the same time?
- Is the DB itself optimised, with indexes and everything?
- Is disk I/O out of the question or is there a significant disk wait during the time when the CPU hits 100%, or shortly before that moment?
- What does "top" show when this happens? Is PHP spiking its CPU usage or is MySQL showing on top?
My opinion is that you should hire someone and get them to profile the whole thing if that is possible, as the graphs above don't provide any clues. And I don't think that you have a problem with bots either.
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