Streaming formats maintain timestamps for each frame, whether audio or video, which govern when the player ought to present them. Those non-zero big start times usually occur when a snippet is cut out from a longer video and the tool used does not reset the timestamps. Although if this FLV was recorded on its own, then it's strange.
In any case, running the ...
Be careful ISO/IEC 14496 Part 10 is a video coding standard also called AVC or H.264. The associated file format is ISO/IEC 14496-15 which is based on ISO/IEC 14496-12 (aka ISOBMFF).
A standard is the result of a compromise between different people, different companies, different application domains and different scenarios. Indeed the ISOBMFF is very wide ...
AVC intra and MJPEG are intra frame (keyframe) encoding only, each frame in your video will be encoded as a whole. Distance between key frames is a parameter for inter frame compression, where some pictures are encoded either as a whole (i-frames a.k.a. keyframes), and pictures between keyframes are encoded in reference to other frames either before (p-...
The good old ffmpeg gives you the power ;)
ffmpeg -i input.avi -c:v libxvid output.avi
You can obviously fine tweak the command line with bitrate, audio in mp3 and so on.
ffmpeg -i input.avi -c:v libxvid -b:v 512k -c:a libmp3lame -qscale:a 4 output.avi
This is for a single bitrate use, where you use a constant bitrate for every frame.
Otherwise you ...
If it's just a matter of which FOURCC to assign, and not the codec, then choose MP4V since it has broader compatibility, and the tag can always be changed later, using a tool like ffmpeg i.e.
ffmpeg -i input.avi -c copy -map 0 -vtag DX50 tagged.avi