Hello, my name isXXXXX will stay with you until the problem is resolved.You really need the original disks that came with the machine so that you can use the hardware test on the disc. There could be many reasons for this the most common in this case is logic board failure, if you have an Apple store anywhere nearby your best bet would be to take it to them and have them run diagnostics. They will do it for free and depending on what status your warranty is in they may repair for free.
You have already done the things that I were going to suggest without having the startup disks.
Sorry I had nothing more for you.
No worries, but I won't be pressing accept....I hope you understand
No problem, I wouldn't either. I do recommend taking it to Apple.