Asynchronization of audio and video

I want to put flash to my iPod, but when I convert them to mp4, sometimes there are audio and video asynchronization, I know sometimes this is common. Cant we use some methods to prove this? I have found a program Moyea SWF to Video Converter Pro which claims using slower-playback-speed to get better effect, yet it is perfect, but can some one tell me details about this? And is there any other method to get a good effect?