I recently built a system intending to play FSX, but sometimes, like with addon aircraft and in detailed cities, the frame rate drops from 30 to about 10-15. My graphics card is MSI TwinFrozrII 6950, so I ran Afterburner in the background and the usage of the card hardly seemed to go about 25%!
Althought the CPU usage jumps around a lot, it is nevery being used 100% and stays around the 50% mark, while playing FSX.
I have 8GB DDR3 1333MHz RAM which nevery climbs above 4GB usage. The only other thing I could think of that could be hindering the game is the HDD, but I'm not sure if that's the problem because I still get low frame rates when slowly taxiing around the airport using 3D Cockpit, which, I'm guessing doesn't need a lot of texture loading and things.
I have installed all the latest updates and service packs for FSX and the addons. I have TrafficX, FSceneX, Captain Sim 757 and CLS 747-200. My settings are quite high, and with other aircraft/locations the computer can handle it fine.
Sometime I run BOINC on my PC and that is able to use all the resources.
I don't know whether it's a software issue or whether my system's just not powerful enough, but surely if the frame rate is dropping it should be using all the PC's resources?!