Welcome to the forum.
First, can you please give details, such as the version of PrE, your computer's OS, the specs. of the Clips, that you have Imported, and are causing the problem?
Next, what you describe is classic "video driver issues."
What is the make and model of your video card?
What is the installed driver version?
Is that the latest from the nVidia, or AMD/ATI Web site?
Good luck, and let us know a bit more, please,