The answers will be in the details.
1. What version of Premiere Elements are you using and on what computer operating system is it running?
2. What is being set for the project preset for your project? (See Edit Menu/Preferences/Project Settings.)
3. What are the full properties of what you are putting on the Timeline, what effects, titles, transitions are you applying to the content?
4. What is the description of your computer resources - installed and available RAM, free hard drive space, processor and clock speed, etc?
Also. When you are previewing the Timeline content in the Edit area monitor, is there a red line (Premiere Elements earlier than 11) or orange line (Premiere Elements 11 and 12) over that Timeline content?
What is you intended export for the Timeline content?
Latest version of QuickTime installed on computer with Premiere Elements?
Running the program from User Account with Administrative Privileges and run with Run As Administrator applied?
Please review and consider. Based on your reply, we can decide what next.