Dark knight issue

when using df polearm my dmg per line is average 90k - 120k per line. When grinding sometimes the dmg flactuates so much to about 50k- 60k per line. This happens quite often. Don't know what is happening but the bug started after consuming the HP and MP vitality potion. Upload an example in the video, first part of video is fine, then after that damage all of a sudden went to about half. http://drive.google.com/file/d/1s-ZMbOjl3KW5Lr2_NpMNM6jUch1xxVtb/view