screen blank

I am using windows98,motherboard is 810chipset celeron 400mhz,64mb RAMand 20GB harddisk.It is working fine inwindows mode but when i watching movie on my system the system oftenly problem .. mean my monitor status show screen blank and monitor powershow red or orange.. then I can not any thing doing some thing on my machine.. therefore i open my system and replug the cable of vga on board then system working..
any one help me …….about that
Note: sound card ,vga and modem are builtin on board ..
thanks Nawaid