I have no idea why the frame rate is dropping. What drivers are you using for the GTX 770? I'm using the 340.52 WHQL drivers (I'm not using the 340.52 drivers because of the problems they cause me in P3Dv2.2 (I don't know if this is still a problem for v2.3 yet).

I get much higher frame rates in Windowed mode than I do in full screen mode, but unlike you, I just maximize the window, I do not stretch it in all 4 directions, nor do I make sure it doesn't touch the task bar.  My screen resolution is only 1920x1080 (24" Acer HDMI LCD Monitor). Although my frame rates fluctuate during a flight (always lower at the departing and arrival airports), they do not gradually drop over the length of a flight. I do not think I have ever used FSX in Full Screen mode in Win 8.1 64bit, so I will have to do a test to see if it performs better.

I haven't flown a flight, in the CS727, that was longer than 2 hours in a long time, but overall, its performance is generally better than the 757, or any of the latest Captain Sim FSX releases (767, 707, 737, 777 L-1011). I do not have this problem with any Captain Sim addons for FSX.