Hmmm, it could be something daft like your AV software doing something in the background. There's no good reason for a FPS drop with an i5, so it had to be down to something running in the background.
<Erf> SkulFuk: gf just made a toilet sniffing joke at me
<Erf> i think
<Erf> i think i hate you
I have 15 fps when i play with the minimum settings :> I have the best gaming laptop dont I? Call me crazy but I think toribash actually looks better with shaders off, so I suggest try that and get used to no shaders.
Your machine is just overheating and dropping power for very little time to cooldown.
After a moment it does it again and again. Dropping frames then good and then drop again.