Hi, my computer specs are:
Windows 7 Home Premium
1TB Harddrive - (850GB Left)
Intel(R) Core(TM) i7 CPU 920 @ 2.67 GHz
64-bit operating system
That looks fine but what kind of video card do you have.
That can be the problem with FPS.
My video card is the GeForce GTX 220.
It has it's limits, that probably the problem.
It can't render graphic intensive objects quickly or handle too much scenery.
Both at the same time (airliner over a heavy forrest or city) pushes it beyond it's limits and you lose FPS or it may stutter or tear out.
Keep autogen medium and no AI if possible, try to keep the slides within reason and you should be able to lock it at 20FPS and hold that.