At first, with terrain effects disabled, I was hitting the ground quite often. I was also having trouble with enemy planes blending in with the terrain.
I increased my Gamma to 1.5 and turned on the bumps. On the slide page I increased the slider that goes from 3 to 0.5 (something to do with how far away the trees are displayed.
Try tweaking your graphics a little. My FR is 70+ most of the time, as compared to 20-ish with everything except shadows.