I've had this problem for a few months now. I thought it started when I installed the Traveller software on the server to add ActiveSync support, but I removed that and the problem still persists.
Basically new instances of "nchronos.exe" keeps spawning (and not ending), so over a period of a few days the server eventually gets drowned in nchronos.exe processes, stops responding and I need to kill Domino. My process count the last time was up at about 330, and when I killed it and restarted the Domino my process count went to 160.
I'm running Domino 8.5.1 with Fix Pack 2.
I don't know if it's relevant, but my Domino server was also acting as a Sametime server. At around the same time that nchronos started playing up sametime also stopped working. None of my users can connect to sametime and in the domino log it keeps telling me "stpolicy.exe" has terminated. I've googled for that and tried a few things, but nothing seems to make sametime work again.