Andy is right. Air Out = Air In. If the Air In is LESS than the Air Out, the missing air must come from somewhere.
Negative pressure could result in the aforementioned situation. The missing air would be pulled into the attic space from the interior of your house. This means that you'll be spending EXTRA $$$ to heat/cool your house only to have it go into the attic and outside!
I think what you may be lacking is Soffit Vents. Soffit Vents are installed under that attic eaves. These vents permit the natural rise of air from the outside, to rise up along the underside of the roof's sheathing to keep it cool. Then, out it goes through the Gable Vents. Check out the 2 pages of this article. Ridge Vents would help too (but you may want to wait until you redo your roof. Ridge Vents can be installed now, anyways ...)