Ah... The "Performing Completition Tasks" hatred message 
This is very related with the email subsystem that your SQL SERVER AGENT is using. it tends to happen when the conectivity is lost with the exchange server while the agent is performing an email notification.
Resolution.
1. Ensure you have your exchange server stable.
2. Fix the Outlook profile like so:
-- Remove address book and contacts from it 
3. Cycle SQL Server Agent
Cheers,