Hi Roger,
If I understood rightly, you observe this problem only in a full screen Preview and in the Mini Player (in the main window). Right?
Try to restart Windows to check if it helps to solve this issue.
1. Please can you describe in more details - what exactly you see? Is it skipping frames during video clip playback (choppy playback)? Or some flickering on the monitor?
2. Do you add video clips at HD 1080p quality, or 4K UltraHD?
3. Try to open the Sample Project, included with PTE AV Studio. Browse the Main menu > Help > Open Sample Project.
3rd slide contains a video clip with a slow motion (a pelican bird). Do you see the same issue on this slide in this project?
4. Tell me about a configuration of your PC (graphics card).
If this problem persists, I recommend to manually update drivers for your graphics card (NVIDIA, AMD or Intel).
P.S. Jill, thanks for your advices!