Clip part after out tag is played in rendered video at high speed

I combined a couple of clips to a sequence by

choosing each clip as source, setting in and out tag in the clip, and add it to the sequence by drag and drop.

I added separate audio tracks to the sequence.

After rendering the final video (H.264, MP4), the audio is fine, the video is not.

At the end of a clip, meaning after the out tag of 1st clip, the video doesn't show the content after the in tag of the 2nd clip,
but the rest of the 1st clip (after out tag) at very high speed. Audio continues as expected.

After the final end of the 1st clip, the video continues with the rest of 2nd clip at regular speed, synchronous to the audio.

This happened on two different computers.

A funny thing is that when rendering the video over and over again, this sometimes happened between 1st and 2nd clip,

sometimes between 2nd and 3rd clip, and sometimes between all clips.

Has anybody an idea about the reason? I'm using recently downloaded Premiere Pro CC 2018 on an ASUS notebook.

Thanks