Session hanging with TM REQUEST and plan handle 0x0000 We have a periodic event where lots of connections hang on TM REQUEST (from sys.dm_os_waiting_tasks) and make no progress. KILL on the affected session_id does nothing

The server runs database mirroring (2008R2). Pausing and resuming mirroring fixes the issue when it happens.

The session stuck in this state are unkill'able. The load on the mirror is the usual, not bursty. Once a high number of connections are stuck in this state, the system freezes up and the mirror stops replicating (all queries get blocked).

Anyone seen this before and know what is causing it? There is nothing in the error logs.
TM stands for Transaction Manager. The current command being set to TM
Request means that the thread is currently servicing a DTC (Distributed Transaction Coordinator) request, either to enlist in a DTC transaction, or defect from it, or commit it, or so on. Distributed Transactions are not supported for Database Mirroring because it can't guarantee transaction integrity.  Suppress your DTC transactions and this should go away.