This iMac will freeze unexpectedly with the rainbow roulette appearing. If I close and open the computer right away, it...

Yes, the serial starts with W8439... There has been development.. After trying a lot of different things to eliminate possible options, a colleague of mine told me that it could be the connectors of the hard drive. So in desperation, I have figured I would stuck a spacer (actually a piece of plastic card) between the two connectors and the grey cover surrounding the hard drive. It is now working fine since then. I used to open up the back cover every day when it was freezing, because it would start up most of the time by pressing the internal start up button just beneath the fan. Is it coincidence or my connectors were really loose. And for the HD going, although the Mac Genius Bar dianostic was that there was no problem with it, I had to buy a new one to see if this was really the problem. Since I didn't want to invest in a non-sure thing, I passed that option. thanks