Imac m5521?

I recently purchesed an i mac g3 5--mhz from goodwill, put memory in it and luckily it fired up fine but with a catch. The monitor had fliker at the top third of the screen. I powered it on while inspecting it upsidedown and then the picture was perfect. So i shut it down and put it back together, repowered and same flicker, there is no dial adjustment from what i can see for the vert. and horizontal control, and the picture is perfect on an external monitor on the back vga conector so i do now that the video card is fine, if any one can help that would be great this is a beautiful little machine and it would be easy to turn profit once repaired. Dstorm19@aol.com