iMac — I get a black, grey, light blue or white screen sometimes

This has never happened to me before.

I wasn't doing anything unusual (browsing the web) when suddenly my screen turned all black. I tried to do all possible things, like putting the computer to sleep, hit random keys, or wait a while, but finally I had to restart.

I'm not sure why this happened and was hoping it would happen again.

I started a game and then, suddenly, the same thing happened, but with a grey screen. As you can see, this grey screen also has vertical lines:

http://imageshack.us/a/img267/2347/img7244m.jpg

After restarting the computer, I launched the game again. A minute after starting it, the image of the game started to go crazy, until the screen turned all light blue, with the vertical lines again:

http://imageshack.us/a/img703/7276/img7247t.jpg

A few seconds later it turned white:

http://imageshack.us/a/img15/9069/img7248mp.jpg

Does anyone know what the problem is with my Mac? Why does this happen? I thought it may be related to the game, but the first time when it went black, I was browing the web. I hope it isn't some kind of Virus. I know they're rare on Mac, but you never know.

I'm mentioning this because yesterday or so I downloaded an app, which is the Sendspace Wizard. Basically it's a tool to upload files to Sendspace.com. It's by far the most honest file sharing service I had used, and I've been using it for 5 years now. But you never know... I had to download this app because I have videos of a concert of a friend, it's 700 MB and this app continues broken downloads. One of the files is at 99% but it doesn't list it as finished, it's stuck there. It's very strange, I thought I should mention it as it may be related. Usually I don't like all those uploading serives etc.

Another thing I was thinking of: the Mac was pretty hot. Usually it only becomes hot when I play the game that I have. But this time I wasn't. I heard that when it's too hot, it shuts down itself. So I thought it already did while I was browing (for whatever reason, maybe because the Sendspace tool was uploading all day long already and thus the computer was tired?) and then as I launched the game it turned it off as well because it became even hotter.

Here the specs: iMac 27" Intel Quad Core i5 3.1 Ghz 32 GB RAM, OSX 10.8.2

All help is appreciated !!! Hope it's nothing serious 😟