Problem with mac 24" late 2007 startup

Mar 4, 2010
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Long time reader, first time poster.
sorry for the bad english.
So, basically the problem started a week ago, where my computer crashed whilst playing World of Warcraft. so, I decided to shut it down with power button, but after that, as I tried to turn computer back on, the OS boots up (dock and desktop appears), stays so for a few seconds and is fully functional before it crashes. sometimes it takes 1 second after full boot up to crash, sometimes it stays on for 1 minute before it crashes. how does it crash? sometimes the screen turns to a color (various colors, I saw even Pink), and sometimes the mouse starts to move VERY fast, nothing is clickable or so on, and then mouse INVERSES after 2-3 seconds at huge speed, which makes computer unusable and leads to a hard reset.
If I let the computer off for 4-5 hours, it stays approx. 20-40 seconds more on before the crash.

I tried to boot of windows, it stayed on for 5 mins, and black screen. didn't get past the windows loading screen after a few tries.
Safemod works, I backed up all the data using firewire cable.

Things that I tried to do:

Basically all key combinations, PRAM and so on resets, speeding up iMacs fan with shutting down, unplug, holding power button and plugging, deleting crash, hardware test with CD, reinstalling OSX Snow leopard from Snow leopard disc, fsck with single user mode, disc check with OSX snowleopard disc, deleting windows partition, deleting most of applications, deleting all last DLd things, disconnecting all stuff inclucing replacing keyboard and mouse, taking one ram out and start, taking other ram out and start again, cooling the system down, cleaning fan vents, unplugging the machine, all kinds of resets.

Things that don't work: archieve and install mac OSX (crashes miserably after clicking configure install in OSX CD)

I basically think it can be a problem with VGA and GPU. any toughts?

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