What have you installed other than the game itself? Did you test the game before you started installing anything extra?
This can quite happen if you are using a faulty landclass that you added, or have scenery config errors, or perhaps have a 'texture' folder in an addon folder when you are not supposed to.... Your problem can be caused by a multitude of things. If you have such a fresh new install, I'd recommend taking it all out and starting again. To be honest it's pretty much impossible for a sim forum to help you here without seeing your sim...
When you reinstall, after every addon you put in, you should go and test it runs fine before installing another.
First up though, go and fly in another part of the world. So if you've been installing scenery for New Zealand/ Asia etc, go and fly somewhere like northern Europe, as far from the addon scenery as you can get. If your frame rates return to normal, you should be able to tell it's a scenery in this part of the world giving you problems.
I say this as I've had this problem on a reinstall once myself, and found flying outside of Australia and NZ I was fine. But flying in NZ, Australia, I had SERIOUS frame rate issues. Problem turned out to be a bad install of landclass files that either installed themselves incorrectly (ie me installing them wrong) or the files themselves were faulty..
Good luck
