I suspect this is the curtain of the shuttle speed problem.
I decided to try taking photos with External Flash today. I attached it to my camera and starts taking some photos. The photos look ok at 1/200 suttle speed just quite overexposed. The problem starts here.
1/400 - a quarter of the photo got this big black patch like this. Note that this is not the real photo. This is just an example what it looks like.
1/800 - the patch is even bigger.
1/1000 - oh god... WORSE.
It seems that everything is perfect at 1/200. Does anybody know why this happen and how to fix it?