Premiere Elements 7 on Windows 7 (assumed 64 bit)
What are the video and audio properties of the source media that you have taken to the Timeline workspace Timeline?
What device was used to record this video - camcorder, cell phone, mobile device, other?
What are you setting as the project preset of this project?
Is there a red line over the Timeline content when you are playing that content in the Edit area's monitor?
What happens if you open a new project with the same or different source media? Same problem?
What is the duration of this project and what is your intended export for it?
Let us start here and then decide what next based on the details in your reply.