I have this old computer - P4 2.0ghz, 1.2GB RAM and onboard graphics(no external card), 80GBHDD. It has xubuntu installed on it and it regularly hangs when it takes more system resources for say like graphical programs, too many tabs on firefox etc. It just either hangs the system or shows black screen.

Tested if it was issue of HD but it wasn't because i have used linux livecd and problem is still the same even if I have removed the HD. I have also tested with USB linux (puppy linux and ubuntu linux on 8gb drives). Tested windows XP as well on this HD and results are the same. Tested another HD on this machine, results are still the same with it. System hangs or goes black screen and requires restart.

I thought later it could be thermal heat issue and then applied thermal paste on heatsink but still it fails to work for me. It continues to show symptoms. Another thing which was yet to be tested is changing of CPU fan which was not done because I have not found any fan for old pentium 4 machine in the market. I have to hit online stores (but i am in india and it's hard to find it in online shops which can deliver it to me).

So far I don't see this as HD or Monitor or OS issue because I have tested with other HD and results are the same. So it could be either CPU fan or motherboard? What could be possible issue with the hardware?