An IBM PC running Windows XP home.

No problem in the first year. The video/monitor suddenly went black
intermittently. The computer, apparently, was running alright still. Need
to reboot to get the video back. I bought a new video card (used to use the
port in the mother board), the same happened.

Which part is possibly wrong? Shall I change the motherboard?

TIA

TM, Hong Kong

It sounds like you are having the same problem as me, what are the
specs of your computer (monitor, video card, motherboard, etc).