I've discovered (through painfully puzzling experience) that cleaning out my main box every 18 months or so is mandatory. What happens is the CPU fan blows dust onto the chipset chip nearest the CPU, and it gets caught and collects on that chip's leads (which are very closely spaced). Once enough dust builds up it seems to conduct e/m or static and the system locks up unpredictably. Vacuum the dust away and all is well.