Only guess that I have for what "turned on," would be that you were right at the cut-off point for clean recording and then the lighting level dropped a few Lux and a feature in the camera kicked in, to get you footage, but at the cost of some Video Noise.
I agree with Ann's suggestion to use Neat Video. It works very well, and is worth the $, when needed. Adjusting the Effect will take a bit of tweaking and monitoring (just Render that area, using the WAB) and look closely. Processing time will go up a bit, but I find the results to be very good. If you are doing other Effects, say Highlight & Shadow, Levels, etc., apply those first, and then Neat Video. You might need to go back to those other Effects and tweak them again. Last, look at adding a touch of Unsharp Mask, as the last Effect. Go lightly and study carefully. Remember, with many Effects, their order does matter.
PS - do not know if Neat Video has a 64-bit version out yet, but check that out and do use the trial to make sure that all works fine in CS5 on your system.