Ben, i have had this happen in the past, and it was usually as i had forgotten to plug the guide cable into the autoguider port.
check the PHD settings, sometimes it can turn of guiding on the dec axis if it doesnt get enough movement, but take heart, anytime i have had this error, its always been something really simple, and a slaphead moment
EDIT : I have also had this when trying to guide on a piece of dirt, or a hot pixel, but dont tell anyone !!