The wrong microphone decided to record my voice, so it might be hard to hear me.
This happened to me not too long ago on the Gold Golems, too, but not with any other mob. The only thing I forgot to mention in the above video is that I queued my abilities to kill it. (Had SS, then hit TW right after. SS hit, but then it just... died and TW went on cooldown)
The FPS drop is not likely to be a bug btw, it was probably fraps fiddling with Istaria. It happens occasionally only with fraps.