SQL server timeout after downgrade
The log seems to display an output of the DBCC MEMORYSTATUS, which is what happens when the engine runs out of memory. Did you configure the maximum amount of memory SQL Server can use ? To find out, you can run the following query :

SELECT  *
FROM    sys.configurations
WHERE   name LIKE '%mem%'


For the row for which the column named "name" is valued to "max server memory (MB)", you should see a value different greater than zero.

If it is set to zero, you can easily configure it with the following code, assuming that you server has a couple GB or RAM and is dedicated to SQL Server :

EXEC sp_configure 'show advanced options', 1
GO
RECONFIGURE
GO

EXEC sp_configure 'max server memory (MB)', [your_server_RAM_amount_in_MB_minus_4GB]
GO
RECONFIGURE
GO

EXEC sp_configure 'show advanced options', 0
GO
RECONFIGURE
GO


This takes effect immediately, without the need to restart the SQL Server instance.

If it is not this, did you update all the statistics of all databases on this instance ?