We need more information to help diagnose the issue(s) you are having:
The CPU/processor and graphics card make/models Please quote the make/models names in full. I would suggest you put this information in your profile signature so we do not have to keep asking for it. Download and install MediaInfo https://mediaarea.net/en/MediaInfo and analyse one of the video clips and post the results, see this tutorial on how to setup MediaInfo and analyse a video clip for all the data required. https://www.magix.info/us/tutorials/setting-up-and-using-mediainfo--1279822 (in English) Are the graphics card/integrated GPU drivers up to date?
A screen shot of the Program settings, Device options tab would help.