@Norman-Maclean

Hi

We need more information:

Computer specification, Windows version and program version, see this topic for what is required and please quote processor and graphics card make/model in full. 

I would suggest you put this information in your profile signature so we do not have to keep asking for it.

Are the graphics card drivers up to date?

A screen shot of the Program settings, Display options tab  would help.

Have you changed the source of your video files ie a new camera or mobile device?

What is the video resolution and format you are importing onto the timeline. Download and install MediaInfo and analyse one of the clips that is causing the issue and post the results, see this tutorial on how to setup MediaInfo and analyse a video clip for all the data required.     

The amount of free space on the hard drive(s) you have.

John EB
Forum Moderator