Does your hardware meed the minimum requirements? If you are not sure read the following linked thread:
Dolphin Requirements
Also read the following sticky as well, which may offer some useful tips on using Dolphin:
Frequently Asked Questions (FAQ) about Dolphin
Other things you may want to try is, upgrading video card drivers, tinkering with graphics options in the configuration, ensuring your game is compatible, use a newer build.
Just a quick note, the DirectX video plugin is broke as of revision 3627. This means that even the old DirectX video plugin can not be used anymore until the developers fix this issue.