Premiere Elements 12 Mac
When you talk about rendering are you referring to rendering as in rendering the Timeline to get the best possible preview of the playback of the Timeline content in the Edit area monitor?
What are the properties of this Timeline content? All video files?
Interlaced or Progressive
and/or Brand/Model/Settings for camera which record the source video
Do the properties of your source video match the properties of the project preset that you or the project set? If you let the project do the set based on the properties of the first video drag to the Timeline, please tell us the set details that you find under Edit Menu/Project Settings/General and its Editing Mode, Timebase, Frame Size, and Pixel Aspect Ratio readings even if they look grayed out.
If you are really talking about rendering as in the export sense of the word, please include with the above the export choice under Publish+Share/.......and preset and any customization of it under the Advanced Button/Video Tab and Audio Tab.
Let us start here and then customize the troubleshooting based on your details.