I have a problem with Windows Movie Maker 2012 running on Windows 7 that hopefully someone can help me with.

I have a 43 minute 1920x1080p 60fps mp4 file (about 12GB) that I have added titles/captions/credits/mp3 music. The problem comes when I try to save the movie with the same settings as the original file - 1080p. The program does it's thing and 3 hours later a 7.3GB mp4 file is produced, no error messages. However it cannot be played in VLC and it has no file information in properties/details, no audio or video. So it seems to be a broken file.

Thought maybe a problem with mp4 encoding? Nope, I tried the same project saving at 720p and a 3.3GB file was produced that worked fine.

Thought it might be a limitation of Movie Maker, but tried it on a PC running the same build version but on Windows 8.1 and it worked fine producing a 7GB file.

Other things I've looked at and tried:
SSD has 200GB free.
Made the pagefile limit bigger.
Added more memory from 8gig to 16gig.
Updated GPU drivers.
Disconnected the external HDD which was on continuous backup.

So is it a Windows 7 issue?

Thanks.