My system is acer. it has agp card nvidia ge force.?

in my system acer, i have nvidia geforce agp card. It was working well. but now there is problem with graphics. In the monitor, no signal coming often and sometimes the monitor goes off for someseconds and even it works, there is bad resolution and lines inthe screen. please clear my doubt whether it is caused by agp card and whether have i to change the agp card?