Computer running fast then slow then fast

[FailAtComputer]

Recently my computer has been running slow. When i start the computer its nice and fast when i play games it's fast for about 10mins then it starts to get slow then i alt tab and go back into it and it's fast again for 15secs or so. It has happened before but i somehow fixed it by clean out the dust. This time i have done the same but it did not work. I have used ccleaner cleaned it and i believe it's not overheating because the weather is pretty cold around here. Anyone can help me please.

Some of the info i found on the side of my computer:
Intel Quad core (R) Q8200 2.33GHz 4MB Cache
4GB Dual Channel DDR2 800MHz Memory
1TB SATA 3GB/s Hard disk Drive NCQ
Dual Layer DVD R/RW 20x
NVidia (R) GeForce (R) 9400GT 1G PCI-E Graphics
19 in 1 Media Card Reader
Silent Case Fan
Runnig Windows 7

 

[MindoverMaster]

The outside weather being cool does not mean your CPU/GPU are cool. Download HWMonitor, and show us a screenshot while you are playing your game, to see how hot it really gets.

 

[FailAtComputer]

That was hard xD

 

[FailAtComputer]

if thats too small try Imageshack - monitorp.jpg

 

[brinks]

Your video card is definitely overheating (115C), making it throttle down.

 

[FailAtComputer]

what can i do?

 

[MyGodAL3X]

+1 to brinks, your video card is way too hot

 

[FailAtComputer]

i'm too good at the computer so i open the whole thing and clean it?
and how the **** do i cool it down

 

[MyGodAL3X]

Yeah, you can clean it, that might help a bit.
What is your case?

 

[carnageX]

Just go get some canned air, take the side panel off, and blow the air into the CPU heatsink/fan as well as the GPU's fan. May be easier to work on the GPU if you take it out of the computer (usually 1 or 2 screws to dismount it, and just slide it out of it's slot).