Yes, I have had that problem.  In fact Enterprise Manager will occasionally timeout and fail.  I have to guess that it is contention and locking preventing the select.

I'll bet that the query hint NOLOCK would help see the following:

select * from master.dbo.sysprocesses WITH (NOLOCK)

select * from master.dbo.sysprocesses WITH (NOLOCK)

where status = 'runnable'

order by CPU

desc