Does it improve with just 1 monitor ? It's also strange that your fps is changing so much (from 20-27 or 11-18), which is quite high margin, that shouldn't be especially with a Quad Core. Also, I don't have such difference between the runways and the ramp area. My posted fps were taken on the ramp area, though.
Just a tought: is everything ok with your cooling system ? Because, if something goes wrong, and the CPU temperature starts to rise to unsafe levels, the CPU will automatically throttle down its speed to protect it from overheating.
Another test you can try is the default JFK, using a position similar to my screenshot. If you don't reach at least 41 fps (but you should get even higher than that, because I have a dual 2.66 you have a quad 3.46), something is wrong which at least you would know is not caused by our scenery.