I have found this post (from 1996)and it seems just like mine problem :
[snipped Alice heat/flash description]
Only the Alice corrupted video output bug doesn't crash the computer. In fact, it rarely even corrupts the viewable chip ram! It just causes all sorts of visual effects to be displayed. If you try to snapshot the screen while it's showing this effect, the screenshot looks normal.
It starts with vertical bars of white flashing lines to appear starting from the bottom of the screen, and working their way up. Eventually it looks like you're trying to play your Amiga from inside a flashing jail cell!
Also flickering "strings" start forming to the right of certain objects in the display. Those look like HAM mode bleeding, except they flicker, as well. (These are more likely to cause corruption in screen memory [in other words sometimes they show up in screenshots, and sometimes they'll make an actual change to a picture you're working on in DPaint, etc])
There are certain A1200 revisions that are more prone to this. Every 1D3 A1200 I've used exhibits this problem to a certain extent. It does seem to be heat related, though the Alice doesn't get extraordinarily hot.
Anyhow, while this is a widespread problem, I don't think it's YOUR problem, orange.
That sounds like the problem where if there were any colours onscreen that were full-white, ie 255,255,255, the gfx would get corrupted after a few minutes -- my brother's 1200 had this problem, but mine didn't.
Now THAT'S one I've never heard of! Quite an odd solution, too! What little I know about the real electronics of how computers work would lead me to believe that is an artifact of one of the 'timing errors' that are discussed for the 1D3,1D4,2B motherboards.
