Could this be from spell batching?

IWT is probably not in batching, but everything else in his GSE macro could be. If spell batching is 400ms and you spam your DPS key more than twice a second, your second IWT is connecting before your first spells do.

I have a melee WWWWPa team in Classic now and have not had any problems with it. But there are some differences, the "stutter step" in retail just resets your running animation but the "stutter step" in Classic you come to a complete stop and then start again. But they always run to the target, even if I have been stunned they continue IWT after the stun ends.

Seems to be working OK.