I shot a wedding three years ago with a 7D and a 1D Mark III. As the night wore on, the pavillion was lit with just some strings of 7W nightlight bulbs on strings (glorified Christmas tree lights), plus the DJ's four-color lights on a stand. The 7D really struggled, while the 1D3 didn't care in the least. I suspect you're running into the limits of the 7D systems.

That said, I agree with neuroanatomist's suggestion to add light.