I really have no ideia what might be happening.
We don’t either until you provide some very basic information including source material (frame size/rate, codec, etc.) project settings (project size, frame rate, etc.) and render settings much like you’ve stated. Rather than words, it’s more precise to simply do a screen shot of these property windows and upload them. A screen shot of your timeline can also be of great assistance. For example, here are shots of the three property windows I mentioned.
First your media properties. (Select event. Right-click. Select Properties. Then, Media Tab)

Then, project settings. (Alt-enter)

And finally, render settings. (Select Render Template, then Customize Template)

The more information that you provide, the more likely someone will be able to help.
wwaag