Imac G4 : Blue Or Black Screen Freeze At Start-up


Ever since loading Tiger OS X onto my iMac flat screen G4 I have had frequent bouts of a black screen during start-up- no log-in or anything. I know computer is on, all cords ok, but can't get any visuals or proceed further. I have to force quit and try again. After a recent OS x upgrade that I downloaded from Apple, this problem disappeared, but then I noticed that as the screen flashed to give me my little log-in square, I would see an orange flash of color briefly on the screen. Then one day, at log-in, the user log in square jiggled a bit before settling normally. Then a day later, I was back to unsuccessful start-ups with a black screen I could not get away from. After 4 force quits and restarting with "on" button, computer started and even got as far as showing the "loading OS X" square with the rolling candy cane line in motion, and then suddenly, a flat blue screen... I did put in 256MB RAM before the Upgrade to Tiger OS X- a rather tricky thing to fit in. I alsways completely shut down my computer after work sessions and never left it sleeping, except for now in its blue stillness. I had also recently loaded new software- iLife and iWork and an SD Globe Atlas software from Mackiev. If you send clear steps and don't assume TOO much tech savvy, I will try to fix my computer- or else send it to Apple. Thanks for your help.