Help - Question about Shooting 60D Video

Hi. I'm a novice videographer and am attempting to shoot a greenscreen video with my 60d. It seems that no matter how long I bring the ISO down I get the same amount of grain as if I had it extremely high. This is frustrating as when I key the video, you can easily see the grain and it makes the over all product look like crap. Is there anything else I can do - or am I clearly, with what little information I can give, doing something wrong?

Any tips or suggestions will help. Thank you!

What are your other settings?
Aperture, shutter, etc...

I shoot all video on my 60D at 1920x1080 and 24fps.
In the movie-mode menu I have Exposure set to Manual.
Highlight Tone Priority and Auto Lighting Optimiser are both Off.
ISO set to Auto.
I always leave the Shutter set to 1/50.
I finally adjust the Aperture to suit the lighting/conditions.
Start filming and Manually Focus as required.

Never had any issues with my 60D.

Hope that helps

If you are shooting in anything but manual mode the camera will go full auto during video recording...

Do you have enough lighting? If you are using only a room lamp or an overhead light like a ceiling fan light, that is no where near enough light.