It's kind of the same answer. You had Caligula, Nero/the Year of Four Emperors, and Domitian (and the tail end of Augustus's reign for that matter) during the Pax Romana. You also had significant internal revolts (Boudica's revolt, Judea--3 times, etc.). Those years weren't particularly stable, though they weren't as bad as the Optimates vs. the Populares or the Crisis.

Looking at Europe as a whole and the longest period of peace, I think the correct answer is that the sad, tribal apes that make up the human race can't go 50 years without trying to kill each other. The best we can do is relative peace (the Pax) or stability in a single area (e.g. England was pretty stable and secure most of the 1153 -1455 time period).