lake Why does my Blu-Ray picture go grainy when the scene is dark?

I know it's probably a setting issue, but ... We have a PS3 connected to our LCD TV through an HDMI cable. The games are crystal clear. Blu-Rays are usually clear (especially when the picture is bright and colorful). The problem I'm having is that the picture gets incredibly grainy when the scene is dark or shadowy. Our TV has several pre-set formats (such as "movies," "games," "sports") and it seems to have that problem on all of them. Does anyone have any idea what I can adjust?there