Sql-server – Sudden performance decrease in SQL Server

I've got a problem with a SQL Server 2008 R2 instance. Last week the performance suddenly broke down. All inserts are very very slow. It takes 4-5 times longer than before. On the hardware we found no defects. We had the same problem a month ago, but after one week the problem disappeared as fast as it came.

I'm not an expert on these things but I noted that the performance on all databases on the instance is poor. CPU and Disks are not at their limits, so it shouldn't be a hardware performance issue.

I'll took a look at the cached pages count with this query:

select
       count(*)as cached_pages_count,
       obj.name as objectname,
       ind.name as indexname,
       obj.index_id as indexid
from sys.dm_os_buffer_descriptors as bd
    inner join
    (
        select       object_id as objectid,
                           object_name(object_id) as name,
                           index_id,allocation_unit_id
        from sys.allocation_units as au
            inner join sys.partitions as p
                on au.container_id = p.hobt_id
                    and (au.type = 1 or au.type = 3)
        union all
        select       object_id as objectid,
                           object_name(object_id) as name,
                           index_id,allocation_unit_id
        from sys.allocation_units as au
            inner join sys.partitions as p
                on au.container_id = p.partition_id
                    and au.type = 2
    ) as obj
        on bd.allocation_unit_id = obj.allocation_unit_id
left outer join sys.indexes ind 
  on  obj.objectid = ind.object_id
 and  obj.index_id = ind.index_id
where bd.database_id = db_id()
  and bd.page_type in ('data_page', 'index_page')
group by obj.name, ind.name, obj.index_id
order by cached_pages_count desc

Here I found very high values, but I don't know if it's bad or not. The count is from 201.236 to 1 on the different tables an indexes. Could this be an issue of the poor performance?

Another thing I noted is a very high number of log writes while inserting data. With SELECT INTO ... it's much faster.

Can anyone give me a hint, or can tell me what this cached_pages_count exactly means?

edit:

I also mentioned high waititme during the inserts (up to 300.000 ms) Waittype is CXPACKET and wait ressource is e_waitPortOpen and e_waitPipeNewRow. The high values occur on e_waitPortOpen.

kind regards
Thomas