Problems with yafray rendering: Black areas appear randomly.

Hi! I posted this also to Yafray-forum, but I thought that I might get help also from this forum.

I’m making short video clip with rising company logo. To be short, please see pictures as it tells directly my problem.

Frame 67 (perfect)
http://img56.imageshack.us/img56/5599/0067ot4.th.jpg
http://img56.imageshack.us/my.php?image=0067ot4.jpg

Frame 68 (black areas)
http://img397.imageshack.us/img397/1125/0068kf3.th.jpg / http://img397.imageshack.us/my.php?image=0068kf3.jpg

and the frame 69 is also perfect.

There are also other similar errors in other frames randomly. Please help me, what could cause this and what you experts would recommended to avoid this in the future.

Thanks in advance!