About FPS on mp4 videos [Sony ericsson w580i or w880i]

Hello, I have 2 mobiles phones. SE w580i and SE w880i.
I want to watch some videos on mp4. I converted them videos to 320x240 15fps! At computer the video is ok, but at mobile phone is lose some fps maybe! It freeze at many frames for a while.. Do you know if I could do something for that?

I watch a lot videos on my W580 and didn't freeze. My videos are 320x240@29.97fps, video codec is MPEG-4 bitrate ~480kbps, audio codec AAC LC 256kbps 44KHz stereo. Works great. ;)