Computer boots fine but the monitor is bla...

i have a Samsung CRT monitor. sometimes when i turn on the computer the monitor is blank, but the computer boots fine. when this happens i have checked plugging the vga cable of the monitor to the laptop vga port and the monitor works just fine. sometimes the monitor works and in such a situation, if i shutdown the computer and turn it on again sometimes the monitor is blank. im begining to think this could be something wrong with the mother-board. cannot say for sure when it would work or when it wouldnt. sometimes it works for 2-3 days without any problem and stops working for another 2-3 days.the first time this started i was working with the computer and the whole screen suddenly went white. then i turned the computer off and then turned on again, then the monitor was blank.

Thanks,
Dhanushka