In addition to previous post, I am using a eBox with 1GHz and 1GB RAM at home with pfSense 2.2.6 in nanoBSD mode (a 1GB image on SD card). My 2nd slice still have pfSense 2.2.5 and this behavior happens too (much less times, but still occurs).
The pfSense box is not stressed at all. It handles a bare 2Mbit link with two internet boxes adding bandwidth together.