I'd post on the technical support forum, since you have the issue with one specific toon, even on a fresh install of wow without addons etc. Let their tech guys figure it out.
I was going to suggest, deleting your Cache folder, so it is downloaded fresh, but the clean install without addons has me confused.
If you have a friend who plays, you could try zoning on their system.
Is it only when you box, or do you get the same issue when playing 1x wow?
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