Req: free(ish) tool to join mp4 stream chunks grabbed with rtmpdump

[grelga]

I have some chunks from a videostream saved with rtmpdump. The files are .mp4 .
I can input them to FLV Extract and output separate h264 video files and aac audio files.

I want to join the chunks into one video file without time consuming reencoding. All software I've tried so far has fallen short: either it gives an error when trying to join or joins the file but desyncs the audio in unpredictable ways (not the same desync for each chunk).

I have without success tried to join without desync using these applications, solo and in combination, on both the .mp4 files and on the extracted h264 and aac files:
- mp4box
- flv extract
- GetFLV
- Jaksta
- HD Video Repair Utility
- mpgtx
- copy /b (built in command in windows)
- FlvBind
- ffmpeg

This combination actually works: first use Videoredo quick fix on the chunks. Then concatenate with mp4box. But Videoredo costs almost 100$ which is a bit much when I only need to use a tiny part of the application for a very specific type of job. The trial version of Videoredo has a limitation to only output a part of the file (which is understandable, it being a trial version). The Videoredo quick fix processing of the chunks is very fast.

I look for help on some alternative free(ish) application that can do whatever the Videoredo fix can do but mp4box and the rest can't do.

I can't upload a sample file since the streams are copyrighted programmes. Note: it is perfectly legal to download the streams for personal use where I live but not to redistribute that content. But hopefully this general description might be enough for someone to come to think of an application that might be suitable for this task.

Note: I'm not asking on how to reencode into others formats. That I know. I'm looking only for some fix application that will let me thereafter concatenate successfully with mp4box without reencoding.