Video & Audio merge doesn't work ok

[mourn]

I used VLC to capture my screen and saved it as .mp4. Then I tried to add an external .mp3 audio track and convert all together to .mp4.
When I try to reproduce the result, in general eveything seems to be ok except the first 10 seconds when the image is completely blurred. If I move the play bar backwards I see it ok but if I click on Stop and then on Play again from the beginning I see it wrong.
I've checked that if I cut and remove the beginning of the video, the same happens again with the new first seconds.
Could you help me solve this issue?