We are trying the news screenmodes (which are high resolution: 1024x768 1280x1024 and 720P)
Using those modes most of users have reported gliches.
The issue has been identified as 'timing fix'.
I also had some glitches on my screen, and today my Amiga 1200 is runing very well.
You can see the videos and what I did here : http://ohmygibs.free.fr
Now we are just trying to find an explanation.