The issue made me switch between a 144hz main screen as well as a 60hz secondary display to one monitor at 144hz. The reason for this was that WOW Burning Crusade Classic Gold I didn't want to upgrade my system, and the 60hz monitor I had was due to upgrade.
But while doing so, I made the switch to a larger main screen, which meant that my GPU was now operating an increase in pixels both at that fast a refresh rate. My GPU, which had been fine prior to something as easy as Classic WoW had to do some work. It was not sufficient for the task.
I observed that even with all addons disable, no regardless of the version of DX I selected, whether I turned VSync on or off however is the case if my GPU was at it's limit the game would show a certain levels of flickering. With all the lush vegetation and zones such as Nagrand my GPU could be overloaded to 100%, eventually leading to it being throttled.
It seems that WoW does not like my GPU throttle. Even though my CPU wasn't sweating, I did notice the flickering when my GPU was at its limit. In addition, the fact that addons were enabled increased the frequency of flickering.
As such I limited my in game FPS limit to 60 and things got considerably better. It's pretty annoying since after playing for a while with a higher refresh rate it's obvious that you've going back to 60, but it's far better than the flickering I had been dealing with.
So after changing my settings and limiting my framerate to 60 , I still was seeing a fair amount of flickering. This is what I wrote in my last post. I was thinking that the GPU was the culprit but a small part of me thought that was not the case. In other games in which my GPU was being stretched to the limits, I'd have no problems. It's the right time to upgrade my GPU, so I'll need a new one. (I had planned to do my upgrade to my GPU this year but it's not happening. ...) I am assuming the GPU itself is fine as there are no hardware issues.
My issue is not caused by Windows 10's old DWM, Desktop Window Manager or refresh rate code. Then, it would have to alter the rate of WoW on my primary monitor to match the rate of my GPU, which usually is not more than cheap WOW TBC Gold 144 FPS. My 2nd monitor was fine and was set at 144hz. The DWM would cause a flicker and drop the frames on my primary monitor.