Best free way to split or join .MP4 files?

[MazterJedi]

Hey everyone, first time poster here!

I have been trying to a while now to find a good program to join 3 movies that are in 6 files (movie a, part 1 and 2, etc). I have tried every freeware MP4 joiner I have come across. Even though the individual files play fine in VLC, the joined files ALWAYS are gray screen with audio. Are there any settings or whatever I should check on the files to rectify this?

[pumpysworld]

Also, MP4 Splitter from MP4Tools, but use a video player to scan the video and see where to split, then enter the mark in hrs, mins, and secs in the splitter.