Erratic CPU Usage

I have a strange problem with the CPU load going wild. Some mysqld command peaks out at 99% and then vanishes and comes back again etc. I am staring at top, in ssh right now and it shows the following maxed out at 99% :

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND
17031 mysql 15 0 16072 15M 1924 S 99.9 1.5 0:00 0 mysqld

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME CPU COMMAND
29167 mailnull 15 0 516 260 180 S 99.9 0.0 0:09 0 exim

Sometimes the odd httpd/mailnull process will appear in this list also maxing out at 99%. They only appear for a short while, then they vanish. Does anyone have any suggestions as what could be causing this erratic CPU behaviour? It has only started recently, and no severe traffic increase has occured recently. Thanks in advance...

 

 

 

 

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