Help me understand my bottleneck

I have been trying to run CS:GO on this old crappy laptop, trying to understand the reason why random framedrops happen: I can stand still and do nothing and what happens is that the framerate goes from being a decent 45-50 FPS to unplayable 15FPS. then randomly goes up again. I have been monitoring both my (overclocked) GPU and my CPU, and when the framedrops occur both the CPU and GPU are like 50% on load, what the hell? When the framerate appears to be fluid both are instead on full load or close to it. I know my hardware is shitty but why does this happen?

CPU: Athlon64x2 QL-64

GPU: Mobility Radeon HD3450 (my bet is that it should be this guy's fault, somehow I fail to understand how)

Some more details: staying on the menu screen brings both the chips to full load and stable, ingame we have, on a three segmented time cycle two parts of "dropped" and a part of "playable", what is this crap? All the settings are of course as low as they can get, 720p resolution, but I am fairly sure that the drops occur at any resolution.

Solved: Stupid CPU thermal throttling killed its clock to under 1GHz causing the game to lag. Also big improvement: if you have an old dual core like I do, boot the game with the "-threads 2" command, it makes it run much better.