mp4 video is black and flashes with my puppet every few seconds

Hi guys

I recorded my puppet with the scene set to:     304 sec duration, at 24 fps, with a width and height of 1000px x1300 px.

my finished recording stops at frame 1707 and the time is 00:01:11:03.

so I changed the W X H to 500 x 650 and reduced the duration to 71 sec ( which i'm calculating as the actual length of my movie)  and I left the duration he same at 24 fps.

I then export the scene to Media Encorder, left it at the default setting but my mp4 when it plays is black and you only see the puppet every few seconds.

I'm looked through all the tutorials to see if I can find where I made my mistake but can't figure it out. I did it a couple of times but no difference.

Any ideas??!!

thanks

Denise