Even when all addons were disabled I was able to see that the game could flicker regardless of whether VSync was switched off or WOW TBC Gold on. It happened regardless of the fact that my GPU was at its limits. With all the lush vegetation and zones such as Nagrand the GPU of my computer would be stretched to its maximum and eventually lead to slowing down.
It appears WoW doesn't like my GPU throttle. Even though my CPU was not sweating, I could see the flickering of my GPU when it was buy WOW Burning Crusade Classic Gold running at its maximum. The flickering was made worse by the addition of add-ons.