muxing mp4 & mp3 with ffmpegx sends video weird

[Gibson's Squares]

I tried to mux an mp4 video with an mp3 audio file using ffmpegx.

It turned the video weird in a few places, see example. Also, when skipping through, it generally shows a blocky picture for a few seconds, then clears.

Can anyone explain what's happened and how I solve it?
The command I used was:

ffmpeg -i Video.mp4 -i NewAudio.mp3 -map 0:0 -map 1:0 -vcodec copy -acodec copy Combined.mp4

[Herve]

"modern" videos are made with GroupOfPictures (it's a good way to compress videos whitout too much lose)
A video have to be cut at a begining of a GOP(=first full part of the group). If not, the player reads some incomplete parts first (and display your first screenshot) then reaches the next GOP(it's first full part) and displays the video correctly (your second screenshot))

So this is a normal display, your cut was not accurate, your resulting trim contains some useless part at begining
PS: MPegStreamClip has an option (see its prefs) to automatically cut streams at GOP

bye