well, i do meet this problem, a brand new R720 with i350 NIC, bought at late Jan.2013, with all newest firmware out of factory. then, drop line...again and again. with Dell's 5.0 update 2 ISO, newest drivers on it of coures.
but when i change the system profile form Preformence per Watt, to Performance, the server runs very stable for about 3days...
hope this info help you guys.