Same issue here as well. Usually happens in the late afternoon or evening (EDT). And, although it may only be coincidence, each time it has occurred (at least 5-6 times over the past week and a half) I have had a fair number of tabs open in my internet browser (Chrome), aside from at least 1 or 2 MS Office applications.
Eset Sec. Suite for Mac 5.0.110.0
OS X 10.8.4
Is really becoming quite annoying...