Video freezes at the end of clips.

Hi, I have encountered strange glitch. I have put many short clips into project, with straight cut, no transitions between. Some of clips doesn't follow seamlessly - video freezes for about 1sec at the very end of clip but I can hear audio from following clip and after that 1sec freeze video continues with audio. Therefore I "miss" that 1sec of video from the beginning of the clip. It happens only with some of clips and sometime I can sort it out with slight trimming. But mostly cannot.
What am I doing wrong there?