I’m playing fallout 3 with no lag on medium with 800×600, 8x AA, No Anisotropic Filtering, HDR. My GPU can make High too but my problem here is that suddenly when i play, i get somekind of leak in the cpu. The game goes slow, fast, slow, fast slow, fast. I have to go to the menu and wait a bit and then play again for 2 min and it happens again 
This is because the CPU cache is getting full. CPU may run slow when its over heated.
My pc runs at 70+ degrees. Thats baad. When im not playing its about 67 degrees. So i think its a problem there. For some months ago my GPU died and i bought a new one 8600m gt 512mb, my old one was 8600m gs 256mb witch was slow. But…. the cooling pasta was removed and we didn’t add a new one… CAN THIS BE THE PROBLEM?! Do i need to add a new piece if cooling pasta? The pc is currently in my room witch is cold. Very cold. Im using it as a desktop but only a big screen connected to it and external disk + keyboard & mouse. Will it work to put a fan next to my pc? lol.
I got the problem away for a while while i was running with AA on 4x but it looked horrible?!!? Whats best? Higher resolution with no AA or low resolution with AA?
So, basically my question is:
Why do i get those "slowdowns" ?
How can i reduce/remove it?
Is this caused by heat?
I didn’t have any of those problems while i got my old GPU still working =(
Specs:
Acer Aspire 5920g Laptop
nVidia Geforce 8600m GT 512mb 185.86 driver (because the new one made my pc screen flicker when not connected to big screen)
Intel Core 2 Duo 1,83 Ghz T5550
3 GB DDR2 Ram
Disk 1: 18 GB free space
Disk 2: 36 GB free space
External Disk: 302 GB free space
Virus Free (Checked with AVG Free, Spybot S&D.
Daily Registry cleaning with CCleaner
Used to overclock with rivatuner but removed it after bluescreen problems.
Please read the whole question. I made it as detailed as i could 
It sounds like your problem is definitely overheating. I would be extremely careful right now because overheating does different things to different systems and can do anything from simply causing the system to lag (as it seems to be doing for you) to having the chip literally melt and start causing physical damage to the rest of your computer. Overheating is one of the biggest problems for high-end computers, especially for laptops. One of the first things you should do is make sure you have a stand available that allows for your laptop to cool. One problem people tend to run into often when they use their laptop at a "workstation" of sorts where they plug it into a new monitor, keyboard, and mouse is that they don’t allow for it to have proper ventilation. Best way to do that is to get a stand. Cooling stands can be expensive, but very effective as they keep air flowing underneath the laptop where the overheating normally starts. With the hardware you’re using, it’s probably fine to just get a regular stand just to get it off the table and onto something that will allow for more airflow. I recommend the laptop stand made by Rubbermaid. It’s not something you would think of as it’s not really a computer brand name, but it has great airflow features and wonderful functionality, as well as the fact that it’s only $15 or so at walmart. Next is the more important part. You definitely need a heatsink on your GPU. I believe you called it "cooling pasta". If set up correctly with the goo between it and the chip and all, it will maximize surface area and allow for cooling, specifically for that chip. Even if you were in a deep freezer, you would still experience overheating for really high powered chips if you didn’t have anything else to cool it. Your system has more than enough power to run the game, probably even at high resolution, but it will continue to run slow until you start cooling the GPU properly. I have a similarly built laptop, and i have mine installed with a heatsink, a system fan, and a cooling stand, and i am able to simulate PS2 games with ease under the right settings. If everything is set up correctly, you should be able to run the game at just about any resolution settings.
So to recap, a fan next to your computer will do almost nothing. You want to make sure the bottom of the computer is exposed so you can get some airflow down there, and make sure you set up another piece of "cooling pasta" or some other form of heatsink, and make sure it’s installed with some thermal gel or thermal compound (sold in a bottle and only costs like ten bucks). With that, your computer should run the game fine.