I can almost confirm that is an UPnP driver related problem. If active on startup, it will keep doing some work even if disabled later.
I have the source code of this library, but I am not going to look into this for now.
New release (that I will publish for testing tomorrow perhaps) has got upnp disabled by default, also it has lower cpu usage with an average of 5%, 3% min and 8% max, exception made for some webservice calls that will pump cpu to 90% for a couple of seconds. Also memory is stable to 15%.
It has been running for just 8 hours, so let's see tomorrow if things are still ok.