It was a frame rate issue indeed. Usually I have fps at 7x. When the loss of framerate happens the frame rate drop to 5x. That in effect means within 1 sec period, for may be 0.2 secs you have no frames update at all, then for the remaining 0.8 sec you have like 8x frames/sec, which avg out to 5x frames/sec. It is annoying when the zombies teleported behind me with noclips while I am not watching ,and when I try to shot them I cannot aim well because the frame lost make it hard to track zombies movment. And I know they glitched thou because the wall is intact and there is no other way they can reach me that fast otherwise.