From 30FPS To Below 10FPS Then Back To 30+ Repeatably

Well the A/C works a treat, but the only thing is i get somewhere in the region of 30 fps, then every second (esp when taxxing) the frames drop to below ten, then a second later jump back upto 30+ then back down again to below ten nearly every second? Any idea? Much Appriciate, and wel done PMDG :D