It's all the time. I'm not good with computers. It goes wherever I go. It started when I downloaded the rocket launcher mod. The graphics quality is set to low because I have a bad computer.I get the big shadow glitch after alt tabbing back and forth from the game to other applications several times. Is it all the time for you or does it just occasionally happen? In general it can eventually cause several video card related glitches. Others I get from frequent alt tabbing are certain particle effecters disappearing and CTDs. They all happen very infrequently though and I simply restart the game to resolve the problem,
The latest update for my NVIDIA video card seems to have reduced the frequency as well.