5D2 video records darker than set

Help guys.

I don't do a lot of video and had the studio set to hot lights today for a short bit of video.

Everything looked good on the back of the screen but 2 seconds after the rec button was pressed the video went about 2 stops darker than normal, I could hear the aperture blades closing in.

Phoned Canon but they couldn't advise. Used the 1D4 instead which didn't have this issue.

Any suggestions as to why this is?

what was your f/stop for the shot set to? Did it look like the lens was at that f/stop before or after? Never heard of it being that delayed. Try a different lens?

did you make sure you were using the 5D in manual mode?

Mine does the same thing in certain situations, especially indoors. Never was able to figure it out but you can see the exposure change in the actual recording, not just on the screen.

are you updated to the latest firmware?

I'm not, I haven't gotten around to it but I'll give that a shot and see if that works.