Erratic CPU Behaviour

I have a strange problem with the CPU load going wild. Some mysqld/httpd 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 21360 20M 1944 S 99.9 2.0 0:02 0 mysqld
5712 root 15 0 3988 3988 2736 S 99.9 0.3 0:09 0 httpd
29167 mailnull 15 0 516 260 180 S 99.9 0.0 0:09 0 exim

They only appear for a short while, then vanish. Does anyone have any suggestions as what could be causing this erratic CPU behaviour? It has only started today, and no severe traffic increase has occured recently. Thanks in advance...

Well, this isn't really a hardware problem. I'd ask about it in the mysql forum, probably. I'll move this thread there if you want.