Yeah, I think the only times he ever freaked out were in book 4 and 29. In book 4, he's been dealt the harsh blow of being told his brother is dead, and now Visser Three is going to eat them. This is also back before Visser Three underwent villain decay. Then in 29, when he woke up infested. But the second time is funny.
<No Cassie, he's screaming because he has a Yeerk in his head.> She said dryly.
Book 40, when he faced Gafinilan, was surely a crowning moment of awesome.