I can't help you with what is causing it as there is more than that running, the desktop also uses some. Graphical artefacts are notoriously difficult to diagnose as who is to say when it would reach breaking point and how it might display itself.
I don't use mIRc or Pidgin, so I have no idea what their graphics resource might be, I'm not even sure there is an application that might be able to show what graphics resources are in use and by what program. That tool if it exists would also be using graphic resources.
If it is so infrequent, personally I wouldn't be too concerned about investigation, if it were to be more common then (because of the difficulty in finding what is using graphic resources), I would be reducing the Visual Effects.