Keep in mind that over times psu's do wear out and that not all 400 watt psus are created equally(i have seen a 500 watt psu with less 12 volt power then an old 250 watt one). Check how much power your psu offers to its 12 volt rail(s).
because i currently use a old Thermaltake Xaser III with the side panel missing. and though i have a industrial stanley fan. that pushes immense air or sucks it. it also sounds like a hurricane lol. open side is asking for dust to get all over the internals quickly and that kills performance.