Hi all,
I have been experiencing this same problem with Domino 8.0.1 and BES 4.1.6. For a good 3 weeks the server was behaving, but the past 7 days the server has been restarted 3 times. The Messaging Agent Log (MAGT) shows quite a few hung threads with high WaitCounts and CPU Usage prior to the server going 'pear shaped'.
I see within the last 24 hours RIM have posted a KB article talking about this type of problem, search for KB16097 at the BTSC. (I can't post links yet :/ ).
In my case there are no 4.5 devices, and the Rich Content & Inline Images support was disabled immediately after upgrading from 4.1.5 to 4.1.6.
I have a feeling RIM will come back and say upgrade to MR1.