Why does my iMac not boot sometimes?
Was the mac shipped with 3gb ram? It is possibly not the reason but advice all the same, apple strongly recommends you run ram in identical pairs or individual modules, so a single 1, 2, 4 etc GB stick or 2 x 1, 2 x 2GB and so on, but not 1 x 1gb and 1 x 2gb.