Imac 21,5 mid 2010 RAM issue

[Yako76]

Dear Friends,
I have a problem with my Imac.It start only when one DDR is installed. It keep about 10 seconds to chime, then about 10 Min to work in MAC OS.
If more DDr is installed, than it goes on , black screen, no chime and after 6 second goes off and than again on in loop.
I tried new: Power Supply, Video Card, HDD, RAM.
I order a new CPU, i will see. It is I3 3,06Ghz. May be CPU is bad, or the north bridge in the CPU?
Can anyone help me?

Thanks a lot in Advance!