Well, after 5 hrs of priming you should have already reached any overheating issues, so maybe it's not an heat issue that cause your problems - think it's something wrong with your settings, so you'll have to tweak with fsb term, gtls and so on. i don't have an dfi x48 nor a q9450, so can't help you, sorry!
btw, it seems to me that there will be space for a 80 mm fan instead of a noisy 60mm one, or you may get an antec spot cool to direct airflow where you need it!