Ancient, unsolved high-CPU problem

[Rene Fournier]

For the longest time, I've had a strange problem with MySQL.  
Basically, after a certain amount of time--sometimes a few days,  
sometimes a couple weeks--its CPU usage will go from a steady 20-30%  
to 80-90%. Actual load and number of queries is the same, nothing else  
changes.

If I shutdown MySQL and restart it (not the server), CPU% goes back to  
normal. What could this be?

(Xserve G5 2GHz, 8GB, 3x250GB RAID5, Mac OS X 10.4.11, MySQL 5.0.51a)

...Rene