20" Aluminum iMac - Black screen

Hey guys, so this is my first post on the forum -- apologies if the format is incorrect.

20" Aluminum iMac, 2.66 Ghz Intel Core 2 Duo
4GB memory (came with 2GB and I upgraded it myself)
Running Snow Leopard

Sometimes when I am exiting out of a program (iChat, Firefox, Adium, Microsoft Word)... my screen turns black (can see that it is still illuminated, the screen is still obviously on).

There is no way for me to get my screen back, and if I press the power button on the back of the computer it puts it to sleep or something (does not turn the computer off). The only way for me to get my screen back is by turning off my power bar and rebooting.

This was happening before I upgraded my memory, and it happens 2 - 4 times a week. I am wondering if my iMac is overheating, it gets up to maybe 45C sometimes.

Any ideas?

Thanks

heres a try...

When the ma powers on and you hear the sound, hold the following keys down

command
option
P
R

If you still get problems after that, insert the snow leopard disk taht came with the mac

After you hear the startup sound hold down the following keys

D

This should start the apple diagnostic tests to see exactly what is wrong with the computer

45 degrees C, is nice and cool for the cpu, it isn't even close to overheating.

nice