Please help . imac 20" alu problems, Keep rebooting when programs are used .

Hi people ...

i hope someone can help me cause i cant find any good info on my problem anywhere ...

After i installed win7 and fixed the audio problem these problems came up :

- Rebooted when i tried to use headphones .(but only in windows)

- then the imac rebooted if something hit the screen (just a small toutch like the keyboard when i was putting it down, this happends in os x )

- then the desktop from time to time got covered with a gray overlay with the message u need to reboot your machine .

- then a day later (today) it reboots everytime i try to watch a dvd , run skype , or any other program , (it works fine for like 5 min before reboot)

- some people said it might be overheating problems , so i installed SmcFanControll and the temp was about 50 c , i turned it up and now im under 30 (witch i think is pretty low ) and still the same problems ,

- some people allso said i should try upgrading to snow leopard , so i was going to but i cant cause the mac reboots when im 5 min into the install .
like when i was watchin a dvd ...

btw i tried installing software bundle only and that went fine ...
but still having the same problem with rebooting every time i use some cpu power ..


SO if anyone could help i would be extremely gratefull. since my mac is used for work, entertainment and as a studio pc for my music ...

and i dont have any other source of entertaiment .. so plz plz plz plz help :`(

imac G6 2.4 ghz Intel core 2 duo
4Gb 800 MHz DDR2 SDRAm
OS X version 10.5.7