iMac (late 2009) Hd 4850 graphic issues

Hi guys,

I've got an:

iMac 27-inch (late 2009)

2,8 Ghz Intel Core i7

ATI Radeon HD 4850 512 MB

OS X 10.8.4 (12E55)

When I start playing Quake Live then after a few rounds I notice that the temperature of my mac is rising (maybe upto 53 Celsius).

Then, after a while, when I startup a new round, there is a big change that my screen gets f*** up.

Small yellow, grey and other coloured rectangles will fill my screen/window and if I leave Quake Live every other window will get f*** op to.

If I restart my computer then, most of the time, it'll be alright (at least for a while).

Thoughts:

- 512 MB video memory is maybe not enough. Anyone else playing Quake Live with this configuration?

- The graphic drivers for my HD 4850 where better in (Snow) Leopard. Then I got (the maximum) 125 fps easily. In (Mountain) Lion fps dropped to about 70 fps.

- Turning fan speed on to 2000 rpm will let me play longer

General questions:

- Am I the only person who's suffering from this?

- Does this mean my videocard is on it's way to crash?

- Can my videocard be replaced? (and what are the costs?)

Questions concerning gaming:

--- 512 MB video memory is maybe not enough. Anyone else playing Quake Live with this configuration?

--- Anyone else playing Quake Live on this configuration still has his 125 fps?

Those last questions don't belong here, but may give insight in a heavier use of the graphic processor.

Thanks in advance :-)