This suggests that the fix is merely a side effect of resetting preferences and you have not found the root. Try other troubleshooting. For instance, this sounds more like a caching or memory problem, or a conflict, than a preferences problem, unless it’s related to a specific setting that you are re-setting after the reset (if you follow me). Does a system restart solve the problem? That would suggest a leak or conflict of some kind. Try clearing caches, changing memory allocations, and looking for software conflicts. Does a hard boot (power off completely) make a difference? That suggests a hardware or memory/drive space issue. Make sure all apps and OS are up to date. Run First Aid on all drives. Trash previews and waveform files. Check to make sure all drives, particularly the system drive, have at least 20% slack space. Are you running Encode or AE in background? Any other media apps?
Have you tried creating a new project and importing only the necessary assets, or using project manager to create a consolidated project?