I was on a flight from Seattle to Fresno, and everything seemed fine as the plane climbed to FL350. However, once we reached FL350 the autopilot immediately went into DES mode and began very rapidly descending. The vertical speed was approaching -4000fps and it hadn't stopped increasing the descent rate when I logged off. This is the opposite of the time it continued climbing when it reached the chosen FL, but probably a symptom of the same problem. The autopilot was still set to 35000, and no inputs of any kind had been made to the autopilot.
I have flown several flights where the altitude capture and indeed the entire autopilot performed flawlessly, but this failure to stay at the selected altitude has now happened twice in five flights for me.