External monitor's (DELL U2415) refresh rate locked at 59.88 Hz (NTSC)?
I have the same problem with my MPB 2012. I have a DELL P2418D monitor and connected to Thunderbolt to Display Port. After hard time searching a solution I discovery that cable is the problem. My solution was buy a HDMI adapter from ICZI that supports 4k but my MPB 2012 need 2k and works very well with 60 Hz.