For those having issues, try turning off all background apps & only have the demo running. There are tons of apps that running in the background of computers that may be causing the slowdown. There are apps out there that can shut down all non-vital programs on your computer while leaving those programs that are essential to operating running.
For thoese with duo core pc's - maybe try looking at your processor affinity to make sure the program is using both cpu's. If it is using both, maybe try using just one cpu & see if that helps. Some programs actually run worse on duo core computers. Is FM optimzed for duo core computers? (Sorry, I'm not aware of all the specs for FM)
If I'm not mistaken (and I certainly could be) but doesn't FM 'simulate' other games in the background while playing your game? Maybe this is the cause for some jerkyness???
Please read up online about trying either suggestion (background closing apps & processor affinity). I wouldn't want my suggestions to be the reason someone cpu tanks.