What is wrong with my Alienware m18x, why does it overheat alot?

Okay i have now had this Alienware laptop for a bit over 2 years now, and when i first got it, i have the say i was very impressed with the way it performed, not framerate drops, crystal clear display, great sound, smooth gameplay, i was very happy with my purchase, after about 3 months of purchasing it, it started to randomly shut down and overheat, then i had one of the tech support guys come it saying that it was my fans not working and also my amd cards not working too, so they were replaced by new ones under warranty, then the problem seemed to have gone away. so after about a year, no overheatig, but in the last year i have noticed that it is now doing the thermal shutdown again, but i didnt take as much notice as i was busy rendering and using 3d software packages like Autodesk maya and Zbrush, in which my computer performed fine, no overheating when im 3d mdoelling or rendering, so i thought to myself maybe its just me, but now that i have finished my course, i now have have a bit more time to myself to play games, and now i have noticed the constant overheating alot more, now i think there is something seriously wrong with my laptop.

So i have done a few steps to try and reduce the chances of my computer form overheating, i have:

1) cleaned out all the dust from all 4 fans and also removed the fans to clean out the heatsink radiators.

2) placed my alienware on top of a cooling pad, even soemtimes cranking up the cooling pad fans but with no avail.

3) done the boot up diagnotic test many times, and everything has been given the green tick.

4) used msi afterburner to monitor my fans and temp, so far im getting about 75-80 degrees celcius on all my fans, and they seem to be running according to the msi program.

But after all these things my computer still overheats while playing games, and like i said earlier, rendering, using photoshop, watching dvds, using zbrush dont make the computer overheat, just games. It usually started to overheat after about 5 - 10 mins of gaming, but most of the time the framerate of the game im playing drops down alot then all of a sudden the screen goes black and the computer shuts down.

So here are my Questions:

1) Should i actually check my fans by looking at them while the computer is on to see if they are actually spinning?

2) is it time to repaste the heatsink, cos i have been hearing that sometimes the original paste needs to be replaced.

or 3) is it a bios issue? or do i need to talk to dell support if none of the above are the reason as to why my laptop isnt working properly.

I really need your help good people and i will give you my thanks in advance if you answer my question. Please help me, i want to know what justifies paying a couple of grand on a system that doesnt seem to work properly.

These are my specs:

Dell Alienware m18x laptop

processor: intel core i7 2760QM @ 2.40 ghz

RAM: 16gb

Graphics cards: AMD radeon HD6900 dual graphics cards with crossfire

os: windows 7 64 bit home edition

anything i have missed feel free to let me know.

Thanks again and please help.

Jonathan.