Movies not working!!

I’m having issues with the software on my Apple TV 1st Gen. 
All of the movies I have will work on VLC every one of them on my Mac Pro Desktop.
MP4 and m4v only work via iTunes, but the Majority of my collection are AVI mainly and between AVI, MP4 and M4v there are quite a lot that don’t run or work on the Firecore software for the 1st Gen Apple TV. 
I’m running the latest Apple Software Mountain Lion, and latest iTunes. 
I DL the most recent software that ye have available for the Apple TV 1st Gen. 
Not sure what I can do as some movies will show up and then play for like 2 secs and then hang! and others will give me sound and a black screen. 
I have all the movies on my External HD 2 TB (Western Digital). 
Can you please advise what I can do to rectify. 


Just to clarify there are movies that will play without any issues! if I were to quantify I would say about 1 in 4 Don’t work at al as described above.