...have you suddenly got "something" running in the background, taking up Processor time?...

...
....Windows/Anti-virus updates?
...Anti-virus, Mailware, etc, scans running in the background?
Any of these running in the background, without your knowledge, will slow your overall system down...

...!
Paul....G-BPLF...FS 2004...and a very dubious background!...

...

...!
P.S. Locking your FS 2004 Frame Rate at 20 FPS is a good idea, (for overall "smoothness"), and is especially recommended if you use on-line Multiplayer...

...!
Also, another tip. Make sure that you have sufficient RAM Memory installed, (2GB or more), to prevent the scenery, etc, from spooling from the Hard Drive as you fly...

...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.