We have change the sql server startup user from domain account to local system and it works much better now.
The restart of SQL Server may be the difference.
I have done about 15 upgrades over the last 3 months and encountered some similar performance and stability problems. Can you answer these questions?
Was as stress test of network communication speed performed ?
Are NICs set to "autodetect" ?
What are Disk IO rates (run the virtual io stats data management view)
If using a SAN, was the HBA queue depth setting increased from the default ?
Did you configure Windows and SQL Server for memory management ?
Was Windows file cache size limited? Registry key is HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\SessionManager\MemoryManagement\SystemCacheDirtyPageThreshold
Does SQL Server have lock pages in memory privilege?
Does SQL Server have instant file initialization privilege? ( windows right for "Perform Volume Maintenance Tasks")
Have SQL Server minimum and maximum memory been set?
After a database was migrated to SQL Server 2008, were the following run?
DBCC CHECKDB with data_purity
rebuild of all indexes
update of all statistics
removal of any column statistics where the column is also the first column of an index
removal of any hypothetical indexes
SQL = Scarcely Qualifies as a Language