My iMac OSX 10.6.8 will not boot. After power button is pushed It "Chongs", shows apple logo with gearwheel underneath and after 42 seconds the screen goes blue. Every now and again the gearwheel is active for a few seconds then disappears for a few seconds and then reappears for a few seconds.... and that is it! When the gearwheel is absent I have a mouse cursor which I can move with my magic mouse. I have tried to boot off the install disk. Same result. I have done a smc reset, an NVRAM reset to no avail. It will start up in Targe Disk mode. Motherboard Kaput?