Joined Video File Only Plays To A Point

[br4]

I have nine MP4 files that I want to join to create one file for a movie (all files are the same resolution). I've successfully joined parts one through seven, which plays properly in both Avidemux and Media Player Classic.

When I append file eight to the first seven, the joined, but not saved, file plays OK in Avidemux, but the saved file does not show the video correctly at the beginning of part eight. Part eight alone plays fine.

Anyone have any suggestions on how to correct this problem?

 

[MourningStar]

1. save 1 thru 7 as a single file.
2. append parts 8 & 9 to the single file.