I have a Gateway T-1630 laptop. Specs are as follows:
AMD Turion™ 64 X2 2.0 GHz
ATI RS690T chipset
3072 MB 667 MHz DDR2 SDRAM
ATI Radeon® X1270 graphics (lspci -v reports RS690M, X1200)

I have tried 10.04, 10.10, 11.04, and 11.10. All default Ubuntu installs and no matter what the laptop will just shut off out of know where after heavy use. Example is Pioneer Trail in Facebook which is a Flash Media Game. You can hear the fan increase it's speed trying to cool down the laptop but I guess it can't keep it cool enough because it shuts down. The few solutions I have tried so far
1. Took off panel and cleaned out fan and vents
2. I've tried to use CPU scaling and set it to "conservative" or to just stay 800Mhz
3. The bios is updated with the most recent bios per Gateway website (90.03)
4. Tried using Google Chrome Browser because maybe it was a firefox memory leak issue but it still shut off when playing pioneer trail in Chrome Browser also.

I am at my wits end here. Can someone please help me? Is it the radeon open source driver that causes it to overheat? Should I try to install fglrx? I love Ubuntu and want to use ubuntu on this older laptop but obvisouly I can't if it's just going to shutoff when trying to use it under heavy stress. The battery doesn't hold juice anymore, so it's always plugged into wall, does that make a difference?