i dont know if this posted first time so sorry if it did

i have an imac g5 20" 1.8ghz

was working but would shut off at random then come right back on all caps are good

then it shut off and started up could hear start up chime but no image on screen

i tried resetting pram by holding ctrl opt and p + r wont reset pram either

lights 1 and 2 come on on the led on logic board so i know the logic board is getting power lights 3 and four dont come on

any idea what to try

thanks
roger