i am running the same CPU as you and i have over clocked mine to 3.0ghz to get a decent frame rate out of it.
the fluctuation would mainly be caused by 2 things:
1. more happening on screen. very simple, more stuff on screen is more work for the PC. your frame rate will drop if the amount of stuff being rendered is greater then the power of your system.
2.background app. sometimes i can be mid game and my antivirus can kick in which kills my frame rate. make sure that when you are gaming you are not downloading updates and or and cpu intensive tasks.
the CPU is def the weakest link and unless over clocked would be worth replacing. check out the overclocking stickies on this forum for a how to guide and good luck if you take that path. just check your ram, 3 gig is a strange amount to be running on a 775 socket. for the dual channel function on the ram you need either 2x or 4x sticks of ram. 3 sticks will not work with dual channel.