MPCStar supports MP4 format only for mobile devices. Use VLC media player to watch these files. Or install Media Player Classic + CoreAVC decoder to achieve a maximum possible quality. Thanks for the suggested. But I tried using VLC as you suggested. It's still the same. The video freezes and even pixelizes. CoreAVC isn't free so I didn't download it. Do you have any other ideas? Thanks!
Another way is to install K-Lite codec pack. During setup choose ffdshow Video Decoder for H.264 video format. And some other (not ffdshow) for AAC audio. Also you can install those codecs separately, without codec pack. They are free. But K-Lite is better, coz it also installs the latest version of Media Player Classic. Don't forget to uninstall the previous versions first.