Looks that I am not the only one !
I contacted Apple and I had to explain three times the story. They all wanted that I bring back the iMac to an Apple Store, I refused. I just asked to reflash the SMC firmware manually but they didn't know how to do this... The conclusion was that a tech will come on Monday to have a look and reflash the SMC firmware. After talking to the tech to fix the appointment, he explained me that he has no tool to reflash so Apple asked him to change the logic board. This is crazy to replace the hardware if it is just a software issue.
So in the meantime I found something interesting I would like to try. On a forum about issues with an SMC firmware with MacBook Pro, a guy explained that you can reflash the firmware manually if you extract the files needed in the update package (using Pacifist) and if you install Refit. You can have a look on the post number 25 in the link below :
First I will backup all my data with time machine and reinstall the OS in the case of something will go wrong.
I will also have to decide which file to flash because there is two files in the SMC update (k23.smc and k23f.smc). One is probably for i5 logic board and the other for i7 logic board.
If you want to try this, do it at your own risk and please report here your findings.