

OK, I originally thought this was a translucency problem, since it first appeared for soul spheres, lost souls and the like; now it is happening with Demons and Imps too. The screenie is from E4M9; the Demon is real, the Lost Soul is not. The echo monster usually appears a few meters toward the player (from where it really is) if it is behind a wall - as soon as you see it in plain sight, the echo monster disappears and you see the real one instead.
Doesn't happen in software mode. Probably an Intel f***-up, but it'd be nice to know what settings to try, unless it can actually be solved in the engine.