Even with all addons deactivated I observed that the game would occasionally flash even when VSync was turned off or switched on. This was regardless of whether my GPU was reaching its limits. With all its lush vegetation and zones such as Nagrand my GPU would be overloaded to WOW TBC Classic Gold 100%, eventually leading to it throttled.
It seems WoW doesn't like my GPU throttle. Although my CPU wasn't even sweating, I did notice the flickering of my GPU when it was at its limit. And having addons enabled really exacerbated this flickering.
This is why I had to limit my in-game FPS limit to 60 , and everything became much better. It kinda sucks because after playing for a while with a faster refresh rate it's obvious that you've back down to 60 but it's far superior to the flickering I had to deal with.
After changing my settings, however, I was still seeing some flickering. I had thought the GPU was the culprit, but a small part of me thought this was not the case. My GPU could not be able to handle other games. However, I do require an upgrade in the near future, simply because it's now time for an upgrade for it. (I thought about upgrading my GPU but ...) however, I think my GPU is working fine. There are no hardware issues.
The reason I am unsure is that my problem lies in the problem that my version Windows 10 does not have the latest DWM, Desktop Window Manager and refresh rate codes. This would require me to buy WOW Classic TBC Gold alter the rate of WoW on my main monitor to match the rate of my GPU, which is usually not the 144 FPS. Because my 2nd monitor was with 144hz, the DWM would then cause a flicker as it would drop the frames on my main monitor.