Actually a vacuum is much better than a blower. Blower can jam bits of sticky dust into card sockets etc. Make sure to ground properly and clean vents/screens as well. Airflow is king! It is summer in northern hemisphere and a few degrees hotter in ambient temps can cause marginally cooled computers to shut down. In a simplistic fashion you can think of cooling systems in a at their peak provide a constant change in temperature. So if it gets hotter in the room you are going to have hotter cpu temps.
Can you monitor the temperature rise whilst your rendering and see how it compares with normal running. Maybe you could under-clock the CPU and lower the temperature rise that way. Just use the under clock setting whilst you are rendering. For day to day use you will not notice the slower running.