I believe the DIR 655 QOS absolutely can't handling P2P downloading, QOS can do nothing except hang up the router and slow down the network , So just disable the QOS, you can got a very solid router.
Good news, today received replaced 655 from Dlink office, seems everything is OK,
Just download 4 torrent files meantime in office (100M connection), total 3300 sessions, 250 ESD connections, around 5M/s speed,QOS enabled, router is stable, no ping delay and packet lost under heavy download,
so the conclusion is QOS is applicable for heavy torrent handling no problem,
my old DIR 655 is a defective,
So sadly I waste 3 days on this,