New Sony TV as a second monitor

telling us you have an imac is not enough info

Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card

Does the sony show an extension of your desktop before you launch premiere?  

If you have a relatively recent imac with usb3 or thunderbolt, you might consider getting a intensity pro interface from blackmagic to feed the sony monitor.  Nice little gizmo...  not horribly expensive.

Try updating or rolling back your graphics driver directly from the video card manufacturer’s site.