You may have 1080P videos on your computer, but the iPad can't play them-in fact, iTunes won't even let you copy them to the iPad! The easiest solution? Downrez them to 720P using an app like Handbrake.
Joel gives you a full tutorial on that here.
You can also stream music and videos to the iPad with StreamToMe.
"iPad Tips and Tricks: Downrez 1080P HD Videos,Or You Can't Watch Them "
Reference : top-ipad-video-converter.com