Does it do this in the preview window, or after the project is exported? Three things to try...
In Videopad click OPTIONS in the menu bar. Under the EDITING tab, toggle the "use hardware-accelerated video effects" checkmark to the other state.
Under the DISK tab, press the Clear Unused Cache Files button.
Check with the chip or card manufacturer of your video card/adapter for an updated driver.
If none of these measures helps, please share your project so someone can check it out...
Click FILE menu|SAVE AS PORTABLE PROJECT. Upload ALL files that result to DropBox, Google Drive, MS OneDrive or the like. Make the link public and post it here.
If there are numerous files, zip them first, then upload the zip file.