Strange performance issues

Hi. I'm recently completely lost with this weird performance issue of my Asus g73sw. It might have been a couple months since this problem started.

Specs:
-i7-2630QM
-8gb 1333mhz
-NVIDIA GeForce GTX 460M 1.5gb
-Win7 Home premium x64 (fully up-to-date)

I would like to underline that the issue I'm having occurs time to time, which doesn't let me understand the source of it. I do not change anything but it happens for a while and then it gets back to normal, and so on.

I'm playing a (demanding) game called Hawken regularly, where my laptop gives me quite fine performance, usually a decent 90+/- FPS (on lowest settings). But when this "issue" I've been talking about happens, I get about 40 FPS if not lower. Like I said, it's totally random to me. And it lasts for a few days mostly.

And It's not only low fps, when the computer is running slow; it also gives me kind of sluggish/stuttering/laggy feeling.

It's like, I turn on my laptop and it has a chance to run at the high speed, or the low speed. And I know it will last for a while.

The FPS in that game I mentioned above is not the only indicator of the problem that is there for sure. For example: There also is low speeds on web pages, like when I hover my mouse cursor over some links on a page; they flash with a clearly visible latency where they should be flashing instantly when I hover the mouse over them. Even on my 10 years old computer I was able to do that!

Anyways, I think I made the problem clear enough. I have been researching since this problem showed up, and would like to give you information about the solutions I tried since then.

-Have cleaned the dust away regularly (once in 2 months or so). But the laptop is 2 years old now and might be in need of a repaste.
-Been checking the tempratures real-time. They're 40 cpu - 55 gpu when idling (in celcius), gets to 70/80 cpu - 80/85 gpu when on load (Gpu doesn't go higher than 85) And these are recent tempratures, 30+ celcius here everyday lately, in summer. (I should note that, tempratues are about the same when my PC runs fast or lower. They don't really differ)
-Tried most recent drivers for GPU, Intel chipset, Network adapters, Sound drivers.
-Tried flashing most recent BIOS and a couple vBIOS'es. (Performance got fluent and nice once again while trying different GTX460m vBIOS'es, but I think it was just luck, because it went sluggish later on again)
-Tried removing nvidia drivers completely by using DriverSweeper and ccleaner, and then installed new ones.
-Scanned for viruses, malwares. (Been using Malwarebytes AM, ClamWIN AV, Spybot S&D)
-Defragged HDD
-Disabled Core parking of CPU
-Checked background windows services running
-Ran DPC Latency checker, located at here.

There is however an interesting situation about that DPC latency results, some of other g73 owners seem to be complaining that they get really bad results just like me. Our common experience is that;
-When you disable 460m over device manager, DPC gets back to reasonable values.
-Yet, they claim that it is fine with 460m enabled and high DPC values, because there is nothing to do. As mentioned here.
But still, I'm not sure if this is relevant with my issue. Because DPC values are still high when my laptop runs fine.


I think this is all I can tell you about this issue.
Thanks for your interest
And sorry about my second language