I have been having this issue for quite some time, have tried multiple tweaks, such as bufferpools, affinitymask and the like, etc. On takeoff and landing, seems to happen every so often my frams drop from stable 30fps to 10fps then assing a certain altitude/distance from the airport it climbs back up to 30 again. I am currently running an AMD FX-8370 4.0GHz processor with a water cooling unit on it so its keeping at about 30C, I am currently using a Nvidia GT 545 3GB graphcis card, I did have a R9 270X in the system but due to some shader/driver issues I have gone back to the GT 545 for now. On both cards I have experienced this issue. I did use AMD's Clean uninstall utility to ensure that all of the AMD graphics card drivers we're removed properly. I also run ORBX Global, however it did this issue prior to insalling this.
Any help would be greatly appreciated!