FPS is effected by your computer processing the game and other programs running in the background. If your resources are being used up excessively, it is the computer and not the game. Try getting the desktop gadget to monitor your resource usage.
For Windows 7: right-click the desktop, click gadgets, right-click on the CPU monitor, click add. (Big is CPU, small is RAM)
For Windows 8: Go to: http://download.cnet.com/Windows-8-CPU-Meter/3000-12565_4-75915865.html Download it, right-click to set it above everything. (Big is CPU, small is RAM)
If your RAM spikes, it might be an easy fix depending on your motherboard's limitations (maximum allowed RAM and amount of available RAM slots). If it's the CPU, you'll need to add a new one altogether. This can be hard because the older motherboards don't allow for CPU removal and if you are able to swap the CPU, you need to get one for a desktop, not a laptop!
Hope this was helpful :D