Hi Mayer
I wonder if you ever found an answer to your problem?. I have the same iMac and it's displaying the same symptoms. I described the problem to a Mac fix it company and they said it could be an Inverter issue or even worse, a logic board fault which they said was unlikely. They offered to sell me a Inverter board but I don't want to spend unnecessarily and find I do need a new LCD. So can you let us know if you ever got the answer?
Thanks