What is the problem with my screen flicoring on mmos. is it the memory on the video card to low?

my screen flickers when playing mmos and only in highly populated areas is it the memory of the video card is not enouph to suport the data or is it something else if you know what it is email me back thank you|||maybe ther is a viruus or somthiing wierd going on put protection software ong (avg is awsome!)