Do you mean rendering as in processing the timeline for better editing performance, or do you actually mean exporting to a new file?
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-PPro Information FAQ http://forums.adobe.com/message/4200840
"Creative Cloud with all the current updates" is not enough information; please report the exact version number displayed in the About Premiere Pro dialog. If it turns out that you're on 7.1, they you might be hitting the bug discussed in this thread: http://forums.adobe.com/message/5826830#5826830.
I'm not clear on what exactly the problem is. Is it that the video appears squeezed in the horizontal dimension during playback? Or did the sequence's dimensions actually get changed?
- What are the Program Monitor's Zoom Level and Playback Resolution?
- What are the sequence settings? (a screenshot of the Sequence Settings dialog would help.)
- Do you have GPU acceleration enabled? If so, please try switching the Renderer to Software Only and see if the problem persists.