Computer Crashes under Linux Live CD But not Windows 7

Hi for a while i have been struggling with trying to get Linux running on my computer, (Specifically Ubuntu 12.04 but have also failed with other distributions such as mint, sabayon and OpenSUSE) I Have Tried the various boot parameters i have seen given as advice such as nomodeset noacpi etc.. pretty much tried all of them alone and in combinations...

The Computer will boot into linux from the liveCD But allways within 1 to 2 minutes, My monitor will go black and the monitor will flash analog/digital in the upper corner then say it is entering sleep mode, Which is what it does when it does not have a signal.. like if i was to turn it on with the computer off.... The Computer power light stays on when it does this, And i see no error messages or warnings before hand....

I Also seem to have this problem when running programs that load before the operating system... Such as trying to run memtest86 off a USBStick to see if that might point out any problems... Or The "ChkDsk" Program to check for hard disk errors that runs during bootup in windows.. same thing as when i boot into linux where it will run for a minute or two then screen will go black..

At first i figured maybe video card drivers but i don't think this is it as i should at least be able to load if i force it to use something like the standard vesa driver or using the nomodeset parameter etc if it was i believe? is this correct? And i dont think i would be getting the error running things like memtest86 etc if it was...

The thing is for whatever reason the computer does not do this at all under Windows 7, I am able to boot into that and have the computer run reasonably stable... I Am stumped as to what the problem is and would greatly appreciate any help!!

System is as follows:

Processor: Intel Core 2 4400 at 2.0 Ghz
Memory: 2.0 GB DDR2
HardDisk: 500 GB (Unknown Brand, Comes UP As "WDC WD500............." In WIndows device manager so i guess Western DIgital)
Video Card: ATI Radeon 4800 Series (Think it is a 4850 to be exact)
Monitor: Samsung SyncMaster 960BF

Once again any help is greatly appreciated, Im lost as to why this runs fine under windows but seems to crash elsewhere, thanks