rendering problems with jpeg files in the exported video

Galone,

I've spent more time converting PNG files to JPG files with the assumption that PRE works something like files directly on the Web—they take up less bandwidth. However, PNG files are more robust and have more data that PRE simply uses, and when you render your video, you'll get better results because PRE doesn't have to deal with the compression data in JPG files. PRE uses a reference to the image file; not the image file itself. So using a PNG file may help.

We'd still like to see a screen capture of your project's timeline.

problem remains after deleting media cache and previews

Delete media cache and preview files and try again.

That is highly unusual!  I've never seen it before and have to assume it is not a program fault but something in your workflow or computer set up.

To try to figure it out, you'll have to give a lot more detail about your project and how you are doing it.