My theory is that what you are seeing is an inconsistent iris in the lens, which is quite normal but does show up as flicker in the film because there is a slight exposure variable. Modern lenese don't have aperture rings, the camera body sends an electrical signal to the lens to stop it down to your selected setting, being electrical you can get fluctuations.
To test why don't you tape over the electrical contact on the lens, or put a fully manual old lens on with an aperture ring and set the aperture to wide open and run a test.
My Canon 7D Blog
