As far as I can tell its just when Im using Sonar.
I know the picture cache within Sonar is suppose to clean itself out when it hits a certain level.
I find myself every so often (maybe every month or so) needing to go in and clean out the folder.
Some of my indications its time are;
the mouse will starting dropping off/going dead
different plugs will get very choppy
the screen tends to freeze & jump
and it all appears to be related to Picture Cache