Hi Michael,
As Simon mentioned, did you check sp_monitorconfig to see if any buckets were fully used? Look for configs where Max_Used is equal to Num_free + Num_active, i.e. at some point all of them were used.
If you can get output from dbcc stacktrace while it is sleeping (waiting), that would give some clues on how we're getting there.
Regards,
Mark Kusma