Notebook randomly starts off

My laptop began acting strangely yesterday. The audio began freezing and cutting in and out. After that, the whole thing froze up. I couldn't ctrl-alt-del or anything. I eventually held the power button until it shut down. When i tried to restart the computer, it went to the press tab to post or del to edit bios. From there it wouldn't move. I tried pressing both del and tab, but nothing worked. I let it sit for a while and eventually got past that screen, but then it froze at the login of windows. I turned it off and restarted. This time it gave me the option to boot into safe mode. I managed to log all the way in and play a game of hearts while in safe mode. When I restarted from safe mode, it froze up again on the first screen saying "press tab to post..." After a long break this time I logged into windows and was able to open AIM and Firefox. After about 2 min, it froze again and I had to hold the power button to restart. I thought it was an overheating problem so i took the lappy apart and cleaned the fan and heat sink. That seemed to have no effect on the laptop. I've also tried booting with a different hard drive and got the same results. This leads me to believe that it is a hardware malfunction. Can anyone give me something to try in order to get it working again?