Static on external monitor when booting up?

When I boot my Mid-2012 MacBook Pro 9,1 the external monitor displays a static image. I can barely make out the login area, but it's still slightly visible. After I enter my password the external monitor goes through several gyrations of screen anomalies, typically random red, blue, and green vertical lines, before it clears up. The whole process takes about 15 seconds.

My MacBook Pro is running Mac OS X 10.13.4 and is connected via a 15' Mini DisplayPort to DisplayPort cable to an LG 34UM95-P external monitor. I currently have the external monitor set to a screen resolution of 2560 x 1080 at 60 Hertz. I've tried a PRAM reset, which had no effect, but have not attempted an SMC reset. I suppose I could also try another cable. Has anyone else experienced this problem and if so, what did you do to resolve it?

For what it's worth, on occasion, I've also noticed some very slight brightness variations on the external monitor when working, almost like the external display is trying to adjust to the ambient light in a room, which doesn't make any sense as the MacBook Pro's clamshell cover is in the closed position. I've turned off Automatic Graphics Switching in hopes that it will help with this issue.

Any help or advice is greatly appreciated.