How did you re-install?
FCP is difficult to uninstall (many components and frameworks). Usually, it’s advised to wipe your drive and clean install the OS and the Final Cut Studio. If you don’t do this, the corrupt files may remain or you could end up with an incomplete installation.
If you have clean installed and you are still having problems, you might consider bringing your iMac back to the Apple Store or wherever you bought it and have it checked out (including the RAM).
If you bought and installed 3rd party RAM yourself, remove it and put in only the RAM preinstalled by Apple and see if the problem goes away. Incompatible or faulty RAM often causes weird issues like this.