My computer screen goes black, and it seems to coincide with overheating. It only happens when I'm watching something online. The screen goes black and I can usually hear the sound for a while before the computer just goes to SLEEP. I cannot PULL it out of the "sleep" mode, I just have to shut it down manually and start it up again. I was wondering if this a video driver issue, or if it was possibly that because I have to keep my laptop plugged in all the time (dead battery) the constant electrical input overloads and overheats the computer and causes this. I have an HP G60 with Windows VISTA. Laptops are more susceptible to overheat than DESKTOP. An additional USB powered external cooling fan will help lower the overheat. However check your laptop for dust and internal fan if its running fine. You could use a free hardware temp. monitoring software to confirm the heat problem such a s SIW or HWMonitor. Check also any power options set and disable it. You could also TEST issue by running safemode.
http://download.cnet.com/SIW/3000-2094_4-10326678.hml
http://www.cpuid.com/softwares/hwmonitor.html