Why do my IMs sometimes come in blank and I have to reboot YIM for it to work properly again?

I have deleted completely all Yahoo files several times and completely reinstalled but continue having this problem. It works fine for a while, but then all of a sudden I will get blank IMs and I have to reboot YIM and then it works fine again for a period of time. Anyone else have similar problems? It used to work fine and then it updated or something.