Same issue! After I upgraded my Mac, my hi-res HTMI-connected Samsung monitor streaked and blipped. The windows also jumped around when it would wake up. I had to unplug the Samsung and now it's worthless and collecting dust. I plugged in an old monitor with a VGA cable and the dock and menu still do not work on the second monitor. Luckily, I have not upgraded at home. ...I use three monitors there. I'm a graphic designer. I NEED multiple monitors. Apple, FIX THIS MESS!!!