What resolution are the source files? How long is the project? What video adapter or card is installed?
Try this: go to OPTIONS | EDITING tab and in the Advanced section at the bottom click the chevron to the right of Default. Choose None (CPU only).
Does that make a difference?

Don't expect highest definition in the preview window, but pixelization is not common.
Export should be clean. Check out the wide variety of export resolutions - even custom settings. 'Auto match' isn't necessarily the optimal choice.

For protection, Save Project File AS routinely, giving it a unique # or name, so as not to overwrite prior saves. Using Save Project Files (no AS) will overwrite. From the HOME tab, that choice is available by clicking the chevron to the right of the Save Project button on the toolbar.
If you take a few minutes to share your project, someone will take a look. The process is easy, quick and can be done privately. Just follow these steps...
Back up --- With your project on the timeline, click on MENU at the top-left. Click FILE|BACK UP PROJECT FILES TO FOLDER. Choose a folder and SELECT FOLDER.
Upload --- Use a free server - Google Drive, MS OneDrive, etc.* - to upload the saved, numbered FOLDER. Do NOT upload the individual VPJ or export file.
Get link --- Get a public link. If using Google Drive click GET SHAREABLE LINK. If necessary change "restricted" to "anyone with the link can view"
Share --- Click COPY LINK | DONE. Paste that link here, or click the folder at the top-right of this forum to message it privately to me. It won't be shared.
* Before uploading, right-click the folder, click PROPERTIES. Look at the File Size to confirm that it's not too big for the free space on the server.