Macbook overheating, fan at 6200 rpm almost always slowing down my laptop dramatically

Hey guys sorry this is my first time making one of these so i hope im doing it right and not repeating anyone else but heres the problem. My laptop over the last 3 months has been overheating within minutes of me opening it. The fan doesnt slowly build up constintly keeping the temperature down, but rather waits until the laptop is (supposedly) boiling and decides to turn on straight to 6200 rpm. Sometimes this happens at 40 degrees, and other times at 80 so its not very consistant. As a result of the fan running so fast almost always (but even when its not running) my computer became very slow. I used to play World of warcraft on it with no problem, always getting a highish framerate, now however, whenever i log in its constantly at around 3-15 framerates per second even if the fan is not on. This is a big problem and i was wondering if there was any way to fix it. I have looked around and some people say clean the fan so i did (making sure not to damage anything and being careful). Other people say to put it onto a cooling mat or something like it, the thing is i never used to have to do this and being a laptop i always lay around with it resting it on my leg or on the couch maybe. I also read another article somewhere, where a man was saying that he had almost the exact same problem and it turned out that there was something wrong with the mother board and apple replaced that for him ad it started working fine.

Now i did get 4gig of ram to replace the 2 gig but the problem started way before this, and infact the only reason i got the 4gig was because the computer was actually running at such a low framrate, even my brother who has a very old macbook (which is very slow) wass complaining about its speed. The computer now works fine with very little lag when its under low stress, but same as before if i try to play any game, even if its a really basic arcade game or something, the computer gives it a slow framerate. The 4 gig was a big improvement from what it was like during the bad faze of the 2 gig, but still isnt completely fixing my problem.

I would be greatful if anyone could help, my laptop is under warranty with apple. I am fairly average with computers so if you want me to list any specs please provide steps on how to find them 🙂.

Thanks a lot!

BTW my Processor is 2.4 GHz Intel Core 2 Duo

and Memory is now 4 GB 1067 MHz DDR3