It's choppy when it plays the h.264 file encoded by JM

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When I use VLC media player to play the h.264 file encoded by JM, the displayed video is choppy, and it seems the same frames are played twice. But ffplayer can play this file smoothly.
When I use a tool to transform the h.264 file into mpeg4 format, it's played normal in VLC media player.
I modified some settings, but the problem still exists. Why does it like this? How can I make it normal?
Thanks.