iMac shutdown when I use Final Cut Pro

Hi, since 1 month I got a random shutdowns of my iMac only when I'm in Final Cut Pro. My computer turn off (Black screen) and I need to unplug - plug the power cord to restarting it.

I can't reset my PRAM too because the iMac shutdown during the process.

My hardware test is ok.

If I reinstall Mac OS El Capitain I can reset the PRAM again but when I return in FCPX the problem come back.

Somebody got this trouble un the past?

Thanks!

François