I bought my HRX217HYA back in June this year, and today I experienced the exact same behavior that Ben48 has described. I've been using the mower once per week without any issues until today. I had started to possibly notice a strange mechanical vibration noise the last few uses but brushed it off as a normal rattle (it was not very noticeable).
The one difference today from other days is the weather was significantly cooler. In my case, the self propel was just crawling along at mabye 1/5 speed, and was barely acceptable when set to full speed. It later started to speed up a bit but when I dropped back down to low speed the same thing occurred.
The belt is tight, the cable moves freely and moves the leaver at the clutch as expected, the cable looks good and is seated properly. It seems as though the problem is internal to the clutch.