May I have your idea about httpd CPU consumption ?

We have 2-3 websites hosted on a linux.os server
Some could believe linux + mysql is not a bad configuration : but not
.. At some period during the day, the http website can not be accessed
at all. http page are very slow (like 10 to 30 seconds to appear).
Read the access log : each access log read, but no solution.
Ensure sql-connect : all sql querries are open and closed properly.

TOP : viewing the processes, when nothing goes well, we notice that the
httpd process consume 15 to 30% of the CPU. We are mas-o-menos sure
that the problem take its source from there.

You experience in that domain will help us so much.

Thanks : many thanks for your answer.