Compaq Pres monitor on off

Hello,
my computer looads fine just that it flashes then dont come on its a compaq pressario when first turning the computer on it the picture on the moniter stays on for about 5 seconds then goes off then comes back on when the windos sign comes up then goes off again then comes back on when it has loaded then goes back of again but then when i turn my monitor off again and turn back on agian it stays on for 5 secs then goes back off any help please thank you