What computer operating system?
Do you have the same problem when you open a new project as you do when you open a saved closed one? If so, target is that saved closed project -
a. what are the properties of the source media on the Timeline
b. how large is the project
c. Are you getting any error messages, such as, "Where is the file _____?"
d. Where are the source media for these saved closed project being saved?
Depending on the details of the above...
1. Does the problem exist with and without the antivirus and firewalls disabled?
2. Is your video card/graphics card up to date according to the web site of the manufacturer of the card?
3. What is the Scratch Disk setup and how much free space is able at their save locations? What is a description of your computer resources?
4. Where are you saving these projects - Local Disc C or external hard drive?
5. Latest version of QuickTime installed on your computer with Premiere Elements and are you running the programs as administrator (from user account with administrative privileges).
Let us start here and then decide what next based on the details in your reply.
Add On...to think about..Troubleshoot a damaged project file | Adobe Premiere Elements